Colored articles and compositions and methods for their fabrication

ABSTRACT

This invention provides improved methods and compositions for achieving material coloration using particle scattering. These coloration effects can be designed to be either highly stable or dependent upon the switching effects of either temperature, integrated thermal exposure, moisture absorption, or exposure to actinic radiation. Articles employing materials with these coloration effects are described. Composition comprise a solid, light-transmitting matrix component having a non-liquid particle scattering colorant dispersed. Articles are produced wherein another solid second matrix component has an electronic transition colorant dispersed therein and the first and second compositions are disposed on one another and optionally interpenetrate each other. Colored articles are produced in the form of fibers, films and molded articles.

This application is a divisional of application Ser. No. 08/535,687filed Sep. 28, 1995, now U.S. Pat. No. 5,932,309.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved methods and compositions for theachievement of material coloration using particle scattering, as well asarticles employing these material colorations.

2. Description of the Prior Art

In the prior art it is well known to color materials using dyes andpigments. Unfortunately, pigment and dye coloration agents suffer fadingeffects due to exposure to ultraviolet light, ozone or bleach. The usualcause of this fading is chemical changes in the colorant. These chemicalchanges alter the electronic transitions of the colorant, therebycausing undesired instability in color. For example, anthraquinone-basedblue dyes fade upon exposure to ozone. Since most dyes contain a bluecomponent, blue-fading causes fading in virtually every color.

It is very common in commerce to color polymers, however, coloredpolymer, such as dyed polymers are also difficult to recycle. Mosthigh-value end-use applications require the separation of recycledplastics by color. However, due to the many colors available, separationby color is rarely done. Instead, most recycling companies separateplastics into a colorless lot and a mixed-color lot. Since dye removalis energy intensive, costly, and causes waste disposal problems for thespent dye, colored plastics remain in the mixed-color lot. Recycledpolymer from this lot produces a marbled polymer or, at best, anoff-green or brown polymer which has limited usefulness. Due to thesedifficulties, many recycling facilities do not even collect coloredplastics. Those that do accept colored plastics often incinerate themixed-color lot. Unfortunately, in some applications, such as carpets,almost all of the material available for recycling is colored. As aresult, these polymers are rarely recycled. Consequently, billions ofpounds of used carpets are discarded in landfills each year, therebywasting valuable natural resources.

The use of chemical colorants, such as pigments or dyes, alsopotentially poses problems related to pigment toxicity and waste streammanagement. Many pigments contain toxic, heavy metals. A wet-dyeingprocess produces spent-dye baths. This dye-house effluent can have anegative environmental impact The range of achievable optical effects isalso restricted if the only colorants are dyes and pigments. A newtechnology is needed which will address fading, recyclability, dye-houseeffluent, and toxicity.

It would be advantageous to provide improved methods of coloration thatprovide switchability from one color state to another. Such colorchanging compositions can be used, for example, for cosmetic purposes inpolymer fibers used for textiles and carpets and for color-changingwindows and displays. Additionally, this type of technology could beused in military applications for camouflage clothing, tents, andmachinery. If such color change is reversibly switched as a consequenceof light exposure, temperature changes, or humidity changes, thenchameleon effects can be achieved for such articles. If the colorswitching effect is a one-time event caused by actinic radiation or hightemperature exposure, the switching effect can be used to providespatially dependent coloration.

Enhancing the value of polymer films, fibers, coatings, and otherarticles by achieving novel optical effects provides a major commercialgoal. One advance in this area is described in U.S. Pat. No. 5,233,465which provides a polymer film having metallic luster resulting from themultiple layering of colorless polymers having differing refractiveindices. These films and derived fibers are presently used for cosmeticpurposes in many applications, such as for product packaging and textilearticles. Another advance is provided by the formation of aparallel-line relief pattern on the surface of a polymer film. This alsoresults in chromatic effects without the use of dyes or pigments. Atechnology of this type in which the parallel-lines relief patternconsists of prisms is described in U.S. Pat. No. 4,805,984. Such polymerfilms are available commercially for solar window and light conduitapplications.

The embossing of polymer films, especially metallized polymer films, toachieve novel optical effects is also well known. U.S. Pat. No.4,886,687 describes non-pigmented coloration as a result of diffractioneffects originating from an embossed pattern having 5,000 to 100,000lines per inch (corresponding to a periodicity of about 0.25 to 5microns). While such embossing provides striking visual effects foreither films or film strips, such effects are difficult to perceive forpolymer fibers having small diameters and conventional fibercross-sections. Also, the embossing described in U.S. Pat. No. 4,886,687is described to be preferably holographically generated by theinterference of two coherent light beams. While such an embossing methodcan provide high reliability of the fidelity of the embossing process,it is also quite expensive.

Novel optical effects in silicate glasses have been achieved usingcolloidal particles of metals. U.S. Pat. No. 4,017,318 describes glassarticles that, after exposure to actinic radiations, can be heat treatedto provide coloration effects because of colloidal silver particles.U.S. Pat. Nos. 2,515,936; 2,515,943 and 2,651,145 also describe methodsof generating colored silicate glasses using combinations of variouscolloidal metals, including colloidal gold and silver. Pearlescentcompositions are also widely used to provide novel optical effects,including color, to polymer articles. These compositions, such asdescribed in U.S. Pat. Nos. 3,087,829 and 4,146,403, provide colorationdue to the interference of light reflected from parallel opposite sidesof platelets deposited on the plate sides of mica substrate particles.This interference-derived coloration process critically depends upon thenearly perfect parallel arrangement of the reflecting surfaces ofplates. Hence, such colorants are sometimes referred to as plateinterference colorants. Due to the many micron diameter of the plates,such particles are unsuitable for the spinning of fibers of the typesconventionally used for textiles and carpets, since the availablepearlescent platelets have lateral dimensions that are comparable to thediameter of the such fibers. As a result, these platelets are eitherfiltered out during the fiber spinning process or they clog spinneretholes.

The pearlescent platelets are preferably aligned parallel to the polymersurface. Without such parallel alignment, the color effect is not asdramatic. Additionally, thick polymer articles are required in order forthe iridescence to be pronounced at the loading levels that can be usedwithout severely degrading polymer mechanical properties.

Christiansen filters have been known for over a century. Such filtersusually consist of particles of a solid in a liquid matrix. Theparticles and the host matrix are chosen so that the wavelengthdependence of the refractive index of the host matrix and particles aresubstantially different and there exist a wavelength at which therefractive index of the host matrix and the particles are equal. At thatwavelength the filter is transmissive and at wavelengths remote fromthat wavelength the light is largely scattered and not transmitted. Foreffective operation at visible wavelengths, such filters should notcontain components that significantly absorb light at these wavelengths.U.S. Pat. No. 3,586,417 shows that the wavelength at which aChristiansen filter transmits can be varied for an optical device byvarying the temperature of the filter. Such variation results from thedifferent temperature coefficients for the refractive indices of thescattering particles and the liquid matrix. Various new methods forproducing Christiansen filters, including some efforts to makesolid-matrix optical devices, are described by Balasubramanian, AppliedOptics 31, pp. 1574-1587 (1992). While Christiansen filters are veryuseful for providing wavelength-selective light transmission for opticalapplications, means for obtaining specially enhanced coloration effectsfor scattered light using the Christiansen effect have not beenpreviously demonstrated. Such enhanced effects for scattered light havecritical importance for the development of new technologies forachieving material coloration.

The present invention eliminates the above described problems of priorart technologies by the use of coloration associated with particlescattering. Materials and methods for modifying and enhancing thecoloration effects of particle scattering are provided by thisinvention.

SUMMARY OF THE INVENTION

The invention provides a composite article comprising a firstcomposition and a second composition. The first composition comprises asolid first matrix component having a non-liquid particle scatteringcolorant dispersed therein, and the second composition comprises a solidsecond matrix component having an electronic transition colorant, dye orpigment dispersed therein. The first composition is either disposed onand substantially exterior to the second composition on at least oneside of the article or the first and second compositions aresubstantially mutually interpenetrating. There exists at least oneincident visible light wavelength and one light angle such that thefirst composition absorbs less than about 90% of the light incident onthe article. The absorption coefficient of the first composition is lessthan about 50% of that of the second composition at a wavelength in thevisible region of the spectrum. The particle scattering colorant doesnot have a highest peak in absorption coefficient that falls in thevisible region of the spectrum. Either (a) the particle scatteringcolorant has a refractive index that matches that of the first matrixcomponent at a wavelength in the visible and has an average particlesize of less than about 2000 microns or (b) the average refractive indexof the particle scattering colorant differs from that of the firstmatrix component by at least about 5% in the visible wavelength range,the average particle size of the particle scattering colorant in thesmallest dimension is less than about 2 microns, and the particlescattering colorant, when dispersed in a colorless, isotropic liquidhaving a substantially different refractive index, is characterized atvisible wavelengths as having an effective maximum absorbance that is atleast about 2 times the effective minimum absorbance.

The invention also provides a fiber comprising a polymer matrixcomponent in which particle scattering colorant particles are dispersed.The particle scattering colorant comprises either a semiconductor, ametallic conductor, a metal oxide or a salt. The particle scatteringcolorant has an average diameter in the smallest dimension of less thanabout 2 microns. The particle scattering colorant has a minimum in thetransmitted light intensity ratio in the 380 to 750 nm range that isshifted at least by 10 nm compared with that obtained for the samesemiconductor, metallic conductor, metal oxide or salt, having anaverage particle size above about 20 microns.

The invention further provides a composite article in fiber formcomprising a polymer matrix having dispersed therein particles selectedfrom the group consisting of ferroelectric, antiferroelectric andphotoferroelectric particles. The invention also provides processes forproducing composite articles.

The invention still further provides a polymer composition comprising inadmixture, a polymer matrix, at least one particle scattering colorant,and at least one electronic transition colorant, dye or pigment wherein(a) either the refractive index difference between the polymer matrixand the particle scattering colorant or the absorption spectra of theelectronic transition colorant, dye or pigment undergoes substantialchange as a result of one or more of a temperature change, humiditychange, an electric field change, pressure change, exposure to achemical agent, integrated thermal exposure, or exposure to either lightor actinic radiation and (b) states exist in which either the averagerefractive index of the particle scattering colorant and the polymermatrix differ by at least 5% in the entire visible spectral region orthe refractive index of the particle scattering colorant and the polymermatrix are matched at a wavelength in the visible spectra region.

The invention also provides an article in the form of a film, fiber, ormolded part comprising a particle scattering colorant dispersed in apolymer matrix, wherein the average particle size of the particlescattering colorant in its smallest dimension is less than about 2microns and wherein either (a) the particle scattering colorant has acoating thereon and the refractive index of the coating differs fromthat of the polymer matrix by at least 10% at all wavelengths in thevisible region of the spectrum or (b) the particle scattering colorantis comprised of a series of layers that differ in refractive indicesbetween adjacent layers by at least 5%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides improved methods and compositions for coloringmaterials using light scattering by particles that are dispersed withinmatrices which are at least partially light transmissive.

The colorants useful for this invention are called particle scatteringcolorants. Such colorants are distinguished from colorants that providecoloration due to the interference between light reflected from oppositeparallel sides or interfaces of plate-like particles, called plate-likeinterference colorants, and those that provide coloration due toelectronic transitions, called electronic transition colorants. Whileparticle scattering colorants can provide a degree of coloration byelectronic transitions, a colorant is a particle scattering colorant forthe purpose of this invention only if coloration depends on the size ofthe particles and there is no significant coloration from theinterference of light reflected from opposite sides or interfaces ofparallel plates. Plate-like interference colorants are specificallyexcluded from the definition of particle scattering colorants. Aplate-like interference colorant is a flat layered material comprising aflat plate layer having a thickness that is between 50 and 1000 nm.

In order for a material to be a particle scattering colorant, it mustsatisfy certain requirements that depend on the invention embodiment.Particle scattering colorants are either absorbing particle scatteringcolorants or non-absorbing particle scattering colorants depending onwhether or not the particle scattering colorants significantly absorblight in the visible region of the spectrum. Absorption is evidenced bythe visual perception of color when particle sizes are sufficientlylarge that particle scattering of light is not significant.

For invention embodiments of a first category, a particle colorant isused by dispersing it in a solid matrix that has a substantiallydifferent refractive index in the visible than that of the particlescattering colorant. For this first category, a particle scatteringcolorant is defined as a material that has either the A or B property asdefined below.

The A or B properties are determined by dispersing the candidateparticle scattering colorant in a colorless isotropic liquid that has arefractive index that is as different from that of the candidateparticle scattering colorant as is conveniently obtainable. The mostreliable test will result from choosing the refractive index differenceof the liquid and the candidate particle scattering colorant to be aslarge as possible. This liquid-solid mixture containing only thecandidate particle scattering colorant and the colorless isotropicliquid is referred to as the particle test mixture. The negativelogarithmic ratio of transmitted light intensity to incident lightintensity (-log(l/l_(o))) is measured for the particle test mixture as acontinuous function of wavelength over a wavelength range that includesthe entire visible spectral region from 380 to 750 nm. Suchmeasurements, can be conveniently accomplished using an ordinaryUV-visible spectrometer. The obtained quantity (-log(l/l_(o)) is calledthe effective absorbance, since it includes the effects of bothscattering and absorption on reducing the intensity of transmittedlight.

The A property is only a valid determinant for particle scatteringcolorants for materials which do not significantly absorb in the visibleregion of the spectrum, which means that absorption is not so large asto overwhelm the coloration effects due to particle scattering. For thesole purpose of the A property test, a material that does notsignificantly absorb in the visible region is defined as one whoseparticle test mixture has an effective maximum absorbance in thespectral region of from about 380 to about 750 nm that decreases by atleast about 2 times and preferably at least about 3 times when theaverage particle size of the candidate particle scattering colorant isincreased to above about 20 microns without changing the gravimetricconcentration of the candidate particle scattering colorant in theparticle test mixture.

It should be understood that the above described ratios of absorbanceswill in general have a weak dependence on the concentration of thecandidate particle scattering colorant in the particle test mixture.Such dependence is usually so weak as to be unimportant for thedetermination of whether or not a material is a particle scatteringcolorant. However, for cases where a material is only marginally aparticle scattering colorant (or is marginally not a particle scatteringcolorant) the above described ratios of absorbances should be evaluatedat the concentration of the candidate particle scattering colorantintended for materials application. Also, it will be obvious to oneskilled in the art that the concentration of the candidate particlescattering colorant in the test mixture should be sufficiently high thatI/I_(o) deviates significantly from unity, but not so high that I is toosmall to reliably measure.

A particle scattering colorant candidate that does not significantlyabsorb in the visible has the A property if the particle test mixturehas an effective maximum absorbance in the spectral region of from about380 to about 750 nm that is at least about 2 times and preferably atleast about 3 times the effective minimum absorbance in the samewavelength range and the average particle size of the material is belowabout 20 microns.

If the candidate particle scattering colorant is significantly absorbingin the visible, it can alternatively be determined to be a particlescattering colorant if another material has the A property and thatmaterial does not significantly absorb in the visible and hassubstantially the same distribution of particle sizes and shapes as thecandidate particle scattering colorant.

For scattering colorant candidates that significantly absorb in thevisible, the B property is also suitable for determining whether or nota particulate material is a particle scattering colorant. Thedetermination of whether or not the B property criterion is satisfiedrequires the same measurement of effective absorbance spectra in thevisible region as used above. The B property criterion is satisfied ifthe candidate particle scattering colorant has a minimum in transmittedlight intensity that is shifted at least by 10 nm compared with thatobtained for the same composition having an average particle size above20 microns.

In another invention embodiment, a colorant is formed when smallparticles, called primary particles are embedded within large particles.For this case, one can determine whether or not the candidate materialis a particle scattering colorant by applying either the A propertycriterion or the B property criterion to either the primary particles orto the embedding particles that contain the primary particles.

These complexities in determining what is a particle scattering colorantdisappear for invention embodiments of the second category, wherein therefractive index of a particle scattering colorant is matched to that ofthe matrix material at some wavelength in the visible. In such cases,any material that has a particle size less than 2000 microns is aparticle scattering colorant. Likewise, the determination of whether ornot a candidate is a particle scattering colorant is readily apparentwhen it comprises a two-dimensional or three-dimensional ordered arrayof primary particles. Large particles of such particle scatteringcolorants will have an opal-like iridescence that is apparent to theeye.

While the above determinations of whether or not a particulate materialis a particle scattering colorant might seem complicated, they are quitesimple and convenient to apply. Particulate materials are much easier todisperse in liquids than they would be to disperse in the solid matricesthat provide the articles of this invention. Also, the measurements ofeffective absorbance required for applying either the A or B propertycriterion are rapid and can be accomplished by conventionally appliedprocedures using an inexpensive spectrometer. Hence, the application ofthese property criteria saves a great deal of time in the identificationof materials (i.e., particle scattering colorants) that are suitable forthe practice of this invention.

In certain preferred embodiments of this invention, electronictransition colorants are used in conjunction with particle scatteringcolorants. An electronic transition colorant is defined as a materialthat has an absorption coefficient greater than 10⁻¹ cm⁻¹ at awavelength in the visible and does not satisfy the criteria for aparticle scattering colorant. Dyes and pigments are also used inconjunction with particle scattering colorants in embodiments of thisinvention. A dye or pigment is defined as a material that absorbs lightin the visible to a sufficient extent to confer visibly perceptiblecoloration. Depending on particle size, a pigment can either be aparticle scattering colorant or an electronic transition colorant. Also,in general, either electronic transition colorants, dyes, or pigmentscan be used interchangeably in invention embodiments.

In use, the particle scattering colorants used in the present inventionare dispersed as particles in a surrounding matrix. These particlescattering colorants particles can be either randomly located orarranged in a positionally correlated manner within a host matrix. Ineither case, intense coloration effects can occur as a consequence ofscattering from these particles. A positionally correlated arrangementof particle scattering colorants is preferred in order to achievecoloration effects that are somewhat flashy, and in some cases providedramatically different coloration for different viewing angles. Suchscattering processes for arrays of particles that have translationalorder are referred to as Bragg scattering. Non-correlated particlescattering colorants are preferred in order to achieve more subtlecoloration effects, which can be intense even for non-absorbing particlescattering colorants.

Since the visual limits of light radiation are approximately between 380and 750 nm, these limits are preferred to define the opticalcharacteristics of the particle scattering colorants for the purposes ofthe present invention. In some embodiments of the invention, theparticle scattering colorants that are preferred have a refractive indexthat is different from that of the host matrix throughout the entirevisible spectral range from 380 to 750 nm and particle scatteringeffects are preferably enhanced using electronic transition colorants,dyes or pigments. This situation differs from that of the Christiansenfilter materials of the prior art that provide matching of therefractive indices of host and matrix materials at least at onewavelength in the visible, and electronic transition colorants, dyes orpigments usually degrade performance. Unless otherwise specified, thedescribed refractive indices are those measured at room temperature.Also, a particle scattering colorant is said to have a differentrefractive index, a lower refractive index, or a higher refractive indexthan a matrix material if there exists a light polarization directionfor which this is true.

The particle scattering colorants, or a subcomponent thereof, should besmall enough to effectively scatter light chromatically. If there doesnot exist a visible wavelength at which a refractive index of thescattering particle colorant and the matrix are substantially matched,this means that the average particle size of such colorants ispreferably less than about 2 microns in the smallest dimension. Byaverage particle size we mean the ordinary arithmetic average, ratherthan (for example) the root-mean-square average. For embodiments of thisinvention where chromatic coloration occurs as a consequence of theexistence of a large difference between the refractive index of thematrix and the particle scattering colorant throughout the visiblespectral region, the average particle size for the particle scatteringcolorants is more preferably from about 0.01 to about 0.4 microns. Inthis case the average particle size in the smallest dimension is mostpreferably less than about 0.2 microns. Especially if the particlescattering colorant significantly absorbs light in the visible, evensmaller average particle sizes of less than 0.01 microns are within thepreferred range. Also, if the particle scattering colorant particles arenot preferentially oriented, it is preferable that the average ratio ofmaximum dimension to minimum dimension for individual particles of theparticle scattering colorant is less than about four and that theparticle scattering colorant particles have little dispersion in eitherparticle size or shape. On the other hand, for embodiments of thisinvention in which the refractive index of the particle scatteringcolorant and the matrix substantially vanishes at a visible wavelength,particle shapes can be quite irregular and preferred average particlesizes can be quite large, preferably less than about 2000 microns. Evenlarger particle sizes can be in the preferred range if the particlescattering colorant contains smaller particle scattering colorantswithin it. This complicated issue of preferred particle sizes fordifferent embodiments of the invention will be further clarified in thediscussion of these embodiments hereinafter.

Instead of expressing particle sizes by an average particle size or anaverage particle size in the smallest dimension, particle size for aparticular particle scattering colorant can be expressed as the fractionof particles that have a smallest dimension that is smaller than adescribed limit. Such description is most useful for the embodiments ofthis invention where the refractive index of the particle scatteringcolorant is much different than that of the matrix at all wavelengths inthe visible. In such embodiments, it is preferable that at least about50% of all particles have a smallest dimension that is less than about0.1 microns.

The matrix in which the particle scattering colorant is dispersed can beeither absorbing or non-absorbing in the visible spectral range. Thisabsorption characteristic can be specified using eitherpath-length-dependent or path-length-independent quantities forcharacterization. For example, if an initial light intensity I_(o) isreduced to I_(t) by absorptive processes after the light passes througha matrix thickness t, then the percent transmission is 100(I_(t)/I_(o)). The corresponding absorption coefficient is -(1/t)ln((I_(t)/I_(o)). Unless otherwise specified, the described absorptioncharacteristics are those for a light polarization direction for whichthere is least absorption of light. For certain applications it ispreferable for the particle scattering colorant to be substantiallynon-absorbing in the visible region. For other applications it issufficient for the particle scattering colorant to not have a highestpeak in absorption peak within the visible. In other applications thatwill be described, it is preferable for the particle scattering colorantto have a maxima in absorption coefficient at wavelengths that arewithin the visible. The latter provides invention embodiments in whichthe particle scattering colorant contains an overcoating layer of anabsorbing material that is sufficiently thin that it produces littlelight absorption.

Light scattering that is not strongly frequency dependent in the visibleregion will often occur as a result of imperfections in a matrixmaterial. One example of such imperfections are crystallite-amorphousboundaries in semi-crystalline polymeric matrix materials. Suchnon-chromatic scattering can interfere with the achievement ofcoloration using particle scattering colorants. Consequently, it isuseful to define the "effective absorption coefficient" using the aboveexpressions, without correction for the scattering of the matrix thatdoes not arise from the particle scattering colorants.

Because of their utility for the construction of various articles forwhich novel optical effects are desired, such as carpets, clothing, wallpaper, draperies, coverings for furniture, polymer molded parts, andcoatings, organic polymers are preferred as matrix materials for thecompositions of this invention. By polymers we mean homopolymers,copolymers, and various mixtures thereof. Various inorganic and mixedorganic and inorganic matrix materials are also suitable for use asmatrix materials for the present invention, such as SiO₂ glasses, andmixtures of inorganic and organic polymers. The principal limitation onthe choice of such matrix materials is that either absorption orwavelength insensitive light scattering are not so dominant that thewavelength-selective scattering (i.e., chromatic scattering) due toparticle scattering colorants is negligible. This limitation means thatsuch matrix materials must have a degree of transparency. Using theabove defined effective absorption coefficient, this requirement oftransparency means that the effective absorption coefficient for thehost matrix in which the particle scattering colorant particles aredispersed is preferably less than about 10⁻⁴ Å⁻¹ at some wavelength inthe visible spectra. More preferably, this effective absorptioncoefficient of the host matrix is less than about 10⁻⁵ Å⁻¹ at somewavelength in the visible, and most preferably this effective absorptioncoefficient is less than about 10⁻⁶ Å⁻¹ at some wavelength in thevisible. Numerous commercially available transparent organic polymershaving lower effective absorption coefficients in the visible areespecially suitable for use as matrix materials for the presentinvention. These include, for example, polyamides, polyurethanes,polyesters, polyacrylonitriles, and hydrocarbon polymers such aspolyethylene and polypropylene. Amorphous polymers having very littlescattering due to imperfections are especially preferred, such as anoptical quality polyvinyl, acrylic, polysulfone, polycarbonate,polyarylate, or polystyrene.

Depending on the intensity of coloration desired, the loading level ofthe particle scattering colorant in the host matrix can be varied over avery wide range. As long as the particle scattering colorants do notbecome aggregated to the extent that large refractive index fluctuationsare eliminated at interfaces between particles, the intensity ofcoloration will generally increase with the loading level of theparticle scattering colorant. However, very high loading levels of theparticle scattering colorant can degrade mechanical properties andintimate particle aggregation can dramatically decrease interfacialrefractive index changes and alter the effective dimensions ofscattering particles. For this reason the volumetric loading level ofthe particle scattering colorant in the host matrix is preferably lessthat about 70%, more preferably less than about 30%, and most preferablyless than about 10%. However, in order to obtain a significantcoloration effect, the particle scattering colorant preferably comprisesat least about 0.01 weight percent of the matrix component, morepreferably at least about 0.1 weight percent of the matrix component,and most preferably at least about 1.0 weight percent of the matrixcomponent. Also, the required loading levels of particle scatteringcolorants can be lower for absorbing particle scattering colorants thanfor non-absorbing particle scattering colorants, and can be decreased incertain embodiments of the invention as either the refractive indexdifference between matrix and particle scattering colorant is increasedor the thickness of the matrix containing the particle scatteringcolorant is increased.

Various methods of particle construction can be employed in thematerials of the present invention for achieving the refractive indexvariations that are necessary in order to obtain strong particlescattering. Preferred methods include (1) the simple particle method,(2) the surface-enhanced particle method, and (3) the onion-skinparticle method. In the simple particle method, the particles aresubstantially uniform in composition and the refractive index of theseparticles is chosen to be different from that of the host matrix. Unlessotherwise noted, comments made herein regarding the refractive indexdifferences of particles and host matrices pertain either to theparticle refractive index for the simple particle method or the outerparticle layer for the case of more complex particles. In thesurface-enhanced particle method, the particles contain an overcoat ofan agent that has a refractive index which is different from that of thematrix. The refractive indices of the surface enhancement agent and thehost matrix should preferably differ by at least about 5%. Morepreferably, this refractive index difference is greater than about 25%.Finally, in the onion-skin particle method, the scattering particles aremulti-layered (like an onion skin) with layers having differentrefractive indices, so that scattering occurs from each interfacebetween layers. This refractive index difference is preferably greaterthan about 5%, although smaller refractive index differences can beusefully employed if a large number of layers are present in theonion-skin structure.

In one embodiment of this invention for the simple particle method therefractive index of the scattering particles is higher than that of thematrix. In another embodiment the refractive index of the matrix ishigher than that of the scattering particles. In both these embodimentsthe difference in refractive indices of the scattering centers and thematrix should be maximized in order to enhance coloration due toparticle scattering. Hence, these embodiments are referred to as largeΔn embodiments. More specifically, in the case where the scatteringcenters are inorganic particles and the matrix is an organic polymer,the difference in refractive index between the inorganic particles andthe organic polymer should be maximized. This refractive indexdifference will generally depend on the direction of light polarization.

In other embodiments of this invention, the refractive index of theparticle scattering colorants are closely matched at least at onewavelength in the visible. In these embodiments it is preferred that (1)there is a large difference in the wavelength dependence of therefractive index of the particle scattering colorant and the matrixpolymer in the visual spectral region, (2) the matrix polymer and theparticle scattering colorant have states that are optically isotropic,and (3) the neat matrix polymer has a very high transparency in thevisible. Such embodiments, called vanishing Δn embodiments, use theconcept of the Christiansen filter to obtain coloration. The size of theparticle scattering colorants are chosen so that all wavelengths in thevisible region are scattered, except those in the vicinity of thewavelength at which the refractive index of the matrix and the particlescattering colorant are matched. This wavelength dependence ofscattering efficiency either provides or enhances the articlecoloration.

Both the high Δn embodiments and the vanishing Δn embodiments providethe means for obtaining either stable coloration or switchablecoloration. In the high Δn embodiments, coloration that is switchable ina desired manner is preferably achieved using the combined effects ofparticle scattering and a wavelength-dependent absorption in the visiblethat is associated with an electronic transition. In the vanishing Δnembodiments, coloration that is switchable in a desired manner can beachieved by effects (light or actinic radiation exposure, thermalexposure, electric fields, temperature, humidity, etc.) that either (1)shift the wavelength at which Δn vanishes between two wavelengths withinthe visible range, (2) shift the wavelength at which Δn vanishes towithin the visible range, (3) shift the wavelength at which Δn vanishesto outside the visible range, or (4) causes a shift in coloration due tocombined effects of particle scattering and chromism in absorption inthe visible that is associated with an electronic transition colorant,dye or pigment. One of the discoveries of the present invention is thatferroelectric, switchable antiferroelectric compositions, andphotoferroelectric compositions provide preferred compositions forobtaining switchable coloration using particle scattering colorants.

Electronic transition colorants, dyes or pigments are especiallypreferred for obtaining switchable coloration for the high Δnembodiments, even when such colorants do not undergo a switching ofelectron absorption coloration. The reason can be seen by considering amaterial (such as a polymer film) that is sufficiently thin thatparticles do not scatter all of the incident visible radiation. In thiscase of the high Δn embodiment, the difference in refractive index ofthe particle scattering colorants and the matrix is large over theentire visible spectral range (compared with the wavelength dependenceof Δn over this range). Hence, changes in the refractive indexdifference between particle scattering colorant and matrix increases theoverall intensity of scattered light, which is generally approximatelyexponentially proportional to (Δn)², but does not substantially changethe wavelength distribution of such scattered light. On the other hand,the chromatic reflection and absorption of an electronic transitionabsorption colorant can provide switchability in the chromatic nature ofscattered light, since the amount of incident light effected by theelectronic transition colorant, dye or pigment can depend upon theamount of light that is not scattered by the particle scatteringcolorant. As an example, one may think of the situation where thescattering effectiveness and thickness of a particle scattering colorantlayer is so great that substantially no light is transmitted through toa layer containing an electronic transition colorant. If the refractiveindex of the particle scattering colorant is then switched so that therefractive index of the particle scattering colorant becomes much closerto that of the matrix, then light can be substantially transmittedthrough the particle scattering colorant layer to the electronictransition colorant layer. Then a switchability in the refractive indexof the particle scattering colorant provides a switchability in thecoloration of the article. This situation is quite different from thecase of the vanishing Δn embodiment, where, even in the absence of anelectronic absorption, an article that is sufficiently thin that it doesnot completely scatter light can evidence a switchability in thechromatic nature of scattered light. This can be true as long as thereis a switchability in the wavelength in the visible at which Δn vanishesand Δn significantly depends upon wavelength in the visible. Thewavelength dependence of refractive index in the visible is usefullyprovided as either n_(F) -n_(C) or the Abbe number ((n_(D) -1)/(n_(F)-n_(C))), where the subscripts F, D, and C indicate the values of therefractive index at 486.1, 589.3, 656.3 nm, respectively. For thepurpose of obtaining enhanced coloration for the vanishing Δnembodiment, the difference in n_(F) -n_(C) for the particle scatteringcolorant and the matrix in which this colorant is dispersed ispreferably greater in absolute magnitude than about 0.001.

Particle scattering colorants and electronic transition colorants caneither be commingled together in the same matrix or mingled in separatematrices that are assembled so as to be either substantially mutuallyinterpenetrating or substantially mutually non-interpenetrating. Thelatter case, where the particle scattering colorant and the electronicscattering colorant are in separate matrices that are substantiallymutually non-interpenetrating, provides the more preferred embodimentsof this invention, since the total intensity of light scattered by theparticle scattering colorant can thereby be optimized. In this type ofembodiment, the matrix containing the particle scattering colorant ispreferably substantially exterior to that containing the electronictransition colorant on at least one side of a fashioned article. So thatthe effects of both a electronic transition colorant and a non-absorbingparticle scattering colorant can be perceived, the thickness of thematrix containing the particle scattering colorant should be such thatthere exists a wavelength of visible light where from about 10% to about90% light transmission occurs through the particle scattering colorantmatrix layer, so as to reach the electronic transition colorant matrixlayer. The preferred thickness of the electronic absorption colorantcontaining matrix layer that underlies the particle scattering colorantcontaining layer (t_(e)) depends upon the absorption coefficient of theelectronic transition colorant at the wavelength in the visible at whichthe maximum absorption occurs (λ_(m)), which is called α_(e), and thevolume fraction of the matrix that is the electronic transition colorant(V_(e)). Preferably, α_(e) t_(e) V_(e) is greater than 0.1, whichcorresponds to a 9.5% absorption at λ_(m). Likewise, for the embodimentswhere the particle scattering colorant and the electronic absorptioncolorant are commingled in the same phase, it is useful to defineanalogous quantities for the particle scattering colorant (which aredenoted by the subscripts s), the only difference being α_(s) for theparticle scattering colorant includes the effects of both lightabsorption and light scattering on reducing the amount of lighttransmitted through the material and α_(s) depends on particle size. Forthese embodiments α_(e) V_(e) and α_(s) V_(s) preferably differ by lessthan a factor of about ten, and more preferably by a factor of less thanabout three. Likewise, preferred embodiments can be expressed for thecase of where the particle scattering colorant and the electronictransition colorant are located in separate phases (with volumes v_(s)and v_(e), respectively) that are substantially mutuallyinterpenetrating. In this case, α_(e) v_(e) V_(e) and α_(s) v_(s) V_(s)preferably differ by less than about a factor often, and more preferablyby a factor of less than about three.

The variation in refractive indices with composition for organicpolymers is relatively small compared with the corresponding variationfor inorganic particles. Typical average values for various unorientedorganic polymers at 589 nm are as follows: polyolefins (1.47-1.52),polystyrenes (1.59-1.61), polyfluoro-olefins (1.35-1.42), non-aromaticnon-halogenated polyvinyls (1.45-1.52), polyacrylates (1.47-1.48),polymethacrylates (1.46-1.57), polydienes (1.51-1.56), polyoxides(1.45-1.51), polyamides (1.47-1.58), and polycarbonates (1.57-1.65).Especially preferred polymers for use as polymer host matrices are thosethat have little light scattering in the visible due to imperfections,such as polymers that are either amorphous or have crystallite sizesthat are much smaller than the wavelength of visible light. The latterpolymers can be obtained, for example, by rapid melt-quenching methods.

Preferred scattering particles for combination in composites withpolymers having such low refractive indices in high Δn embodiments arehigh refractive index materials such as: 1) metal oxides such astitanium dioxide, zinc oxide, silica, zirconium oxide, antimony trioxideand alumina; 2) carbon phases such as diamond (n about 2.42),Lonsdaleite, and diamond-like carbon; 3) other high refractive indexinorganics such as bismuth oxychloride (BiOCl), barium titanate (n_(o)between 2.543 and 2.339 and n_(e) between 2.644 and 2.392 forwavelengths between 420 and 670 nm), potassium lithium niobate (n_(o)between 2.326 and 2.208 and n_(e) between 2.197 and 2.112 forwavelengths between 532 and 1064 nm), lithium niobate (n_(o) between2.304 and 2.124 and n_(e) between 2.414 and 2.202 for wavelengthsbetween 420 and 2000 nm), lithium tantalate (n_(o) between 2.242 and2.112 and n_(e) between 2.247 and 2.117 for wavelengths between 450 and1800 nm), proustite (n_(o) between 2.739 and 2.542 and n_(e) between3.019 and 2.765 for wavelengths between 633 and 1709 nm), zinc oxide(n_(o) between 2.106 and 1.923 and n_(e) between 2.123 and 1.937 forwavelengths between 450 and 1800 nm), alpha-zinc sulfide (n_(o) between2.705 and 2.285 and n_(e) between 2.709 and 2.288 for wavelengthsbetween 360 and 1400 nm), and beta-zinc sulfide (n_(o) between 2.471 and2.265 for wavelengths between 450 and 2000 nm). High refractive indexorganic phases are also preferred as particle scattering colorants foruse in low refractive index phases. An example of a high refractiveindex organic phase that can be used as a particle scattering colorantwith a low refractive index organic matrix phase (such as apolyfluoro-olefin) is a polycarbonate or a polystyrene. As isconventional, n_(o) and n_(e) in the above list of refractive indicesdenote the ordinary and extraordinary refractive indices, respectively,for crystals that are optically anisotropic. The n_(o) refractive indexis for light propagating down the principal axis, so there is no doublerefraction, and the n_(e) refractive index is for light having apolarization that is along the principal axis.

For the case where a high refractive index matrix is needed inconjunction with low index scattering particles, preferred particlescattering colorants are 1) low refractive index materials, such asfluorinated linear polymers, fluorinated carbon tubules, fluorinatedgraphite, and fluorinated fullerene phases, 2) low refractive indexparticles such as cavities filled with air or other gases, and 3) lowrefractive index inorganic materials such as either crystalline oramorphous MgF₂. Various inorganic glasses, such as silicate glasses, arepreferred for use as particle scattering colorants in many organicpolymer matrices for the vanishing Δn embodiments. The reason for thispreference is that such glasses are inexpensive and can be convenientlyformulated to match the refractive index of important, commerciallyavailable polymers at one wavelength in the visible. Also, thedispersion of refractive index for these glasses can be quite differentfrom that of the polymers, so that substantial coloration effects canappear in particle scattering. Inorganic glasses are also preferred foruse in high Δn embodiments, although it should be clear that the hostmatrix chosen for a high Δn embodiment for particular glass particlesmust have either a much higher or a much lower refractive index than thematrix chosen for a vanishing Δn embodiment for the same glassparticles. For example a glass having a refractive index of 1.592 wouldbe a suitable particle scattering colorant for polystyrene in thevanishing Δn embodiment, since polystyrene has about this refractiveindex. On the other hand, poly(heptafluorobutyl acrylate), withrefractive index of 1.367 could be used with the same glass particles ina high Δn embodiment. Relevant for constructing these colorant systems,note that the refractive indices of common glasses used in opticalinstruments range from about 1.46 to 1.96. For example, the refractiveindices of ordinary crown, borosilicate crown, barium flint, and lightbarium flint extend from 1.5171 to 1.5741 and the refractive indices ofthe heavy flint glasses extend up to about 1.9626. The values of n_(F)-n_(C) for these glasses with refractive indices between 1.5171 and1.5741 range between 0.0082 and 0.0101. The corresponding range of theAbbe number is between 48.8 and 59.6. A refractive index that is on thelower end of the above range for commonly used optical glasses isobtained for fused quartz, and this material is also a preferredparticle scattering colorant. The refractive index for fused quartzranges from4.4619 at 509 nm to 1.4564 at 656 nm.

Ferroelectric ceramics (such as the above mentioned barium titanate andsolid solutions of BaTiO₃ with either SrTiO₃, PbTiO₃, BaSnO₃, CaTiO₃, orBaZrO₃) are preferred compositions for the particle scattering colorantphase of the compositions of the present invention. The reason for thispreference is two-fold. First, very high refractive indices areobtainable for many such compositions. For high Δn embodiments, thesehigh refractive indices can dramatically enhance coloration via anenhancement in scattering due to the large refractive index differencewith respect to that of the matrix phase. Second, if matrix and hostphases are matched in refractive index at a particular wavelength in theabsence of an applied field (as for the vanishing Δn embodiments), anapplied electric field can change the wavelength at which this matchoccurs--thereby providing a switching of color state. Alternatively, aferroelectric phase that is an organic polymer can be selected to be thehost phase. If a particle phase is again selected to match therefractive index of the unpoled ferroelectric at a particularwavelength, the poling process can introduce an electrically switchedchange in coloration. Such matching of the refractive index of hostphase and particle scattering colorant can be one that exists only for aspecified direction of light polarization. However, it is most preferredthat the matrix material and the particle scattering colorant havelittle optical anisotropy, so that the match of refractive indices islargely independent of light polarization direction.

Ceramics that are relaxor ferroelectrics are preferred ferroelectricsfor use as particle scattering colorant phases. These relaxorferroelectrics have a highly diffuse transition between ferroelectricand paraelectric states. This transition is characterized by atemperature T_(m), which is the temperature of the frequency-dependentpeak in dielectric constant. As is conventional, we herein call T_(m)the Curie temperature (T_(c)) of a relaxor ferroelectric, even thoughsuch ferroelectrics do not have a single transition temperature from apurely ferroelectric state to a purely paraelectric state. Relaxorferroelectrics are preferred ferroelectrics for use as particlescattering colorants when electric-field-induced switching in colorationis desired, since such compositions can display very large field-inducedchanges in refractive indices. Since these field-induced refractiveindex changes generally decrease as particle diameters become small, theparticle dimensions should be selected to be as large as is consistentwith achieving desired coloration states.

Relaxor ferroelectrics that are preferred for the present invention havethe lead titanate type of structure (PbTiO₃) and disorder on either thePb-type of sites (called A sites) or the Ti-type of sites (called Bsites). Examples of such relaxor ferroelectrics having B sitecompositional disorder are Pb(Mg_(1/3) Nb_(2/3))O₃ (called PMN),Pb(Zn_(1/3) Nb_(2/3))O₃ (called PZN), Pb(Ni_(1/3) Nb_(2/3))O₃ (calledPNN), Pb(Sc_(1/2) Ta_(1/2))O₃, Pb(Sc_(1/2) Nb_(1/2))O₃ (called PSN),Pb(Fe_(1/2) Nb_(1/2))O₃ (called PFN), and Pb(Fe_(1/2) Ta_(1/2))O₃. Theseare of the form A(BF_(1/3) BG_(2/3))O₃ and A(BF_(1/2) BG_(1/2))O₃, whereBF and BG represent the atom types on the B sites. Further examples ofrelaxor ferroelectrics with B-site disorder are solid solutions of theabove compositions, such as (1-x)Pb(Mg_(1/3) Nb_(2/3))O₃ -xPbTiO₃ and(1-x)Pb(Zn_(1/3) Nb_(2/3))O₃ -xPbTiO₃. Another more complicated relaxorferroelectric that is preferred for the present invention is Pb_(1-x) ²⁺La_(x) ³⁺ (Zr_(y) Ti_(z))_(1-x/4) O₃, which is called PLZT.

PZT (lead zirconate titanate, PbZr_(1-x) Ti_(x) O₃) is an especiallypreferred ferroelectric ceramic for use as a particle scatteringcolorant. PMN (lead magnesium niobate, Pb(Mg_(1/3) Nb_(2/3))O₃) isanother especially preferred material, which becomes ferroelectric belowroom temperature. Ceramic compositions obtained by the addition of up to35 mole percent PbTiO₃ (PT) to PMN are also especially preferred for useas a particle scattering colorant, since the addition of PT to PMNprovides a method for varying properties (such as increasing the Curietransition temperature and varying the refractive indices) and since arelaxor ferroelectric state is obtainable using up to 35 mole percent ofadded (i.e., alloyed) PT.

Ceramic compositions that undergo a field-induced phase transition fromthe antiferroelectric to the ferroelectric state are also preferred forobtaining composites that undergo electric-field-induced switching ofcoloration. One preferred family is the Pb₀.97 La₀.02 (Zr, Ti, Sn)O₃family that has been found by Brooks et al. (Journal of Applied Physics75, pp. 1699-1704 (1994)) to undergo the antiferroelectric toferroelectric transition at fields as low as 0.027 MV/cm. Another familyof such compositions is lead zirconate-based antiferroelectrics thathave been described by Oh et al. in "Piezoelectricity in theField-Induced Ferroelectric Phase of Lead Zirconate-BasedAntiferroelectrics", J. American Ceramics Society 75, pp. 795-799 (1992)and by Furuta et al. in "Shape Memory Ceramics and Their Applications toLatching Relays", Sensors and Materials 3,4, pp. 205-215 (1992).Examples of known compositions of this type, referred to as the PNZSTfamily, are of the general form Pb₀.99 Nb₀.02 [(Zr₀.6 Sn₀.4)_(1-y)Ti_(y) ]₀.98 O₃. Compositions included within this family displayfield-induced ferroelectric behavior that is maintained even after thepoling field is removed. Such behavior is not observed for Type Imaterial (y=0.060), where the ferroelectric state reconverts to theantiferroelectric state when the field is removed. However, type IImaterial (y=0.63) maintains the ferroelectric state until a smallreverse field is applied and the type III material (y=0.065) does notrevert to the antiferroelectric state until thermally annealed at above50° C. Reflecting these property differences, the type I material can beused for articles that change coloration when an electric field isapplied, and revert to the initial color state when this field isremoved. On the other hand, the type II and type III materials can beused to provide materials in which the electric-field-switched colorstate is stable until either a field in the reverse direction is appliedor the material is thermally annealed.

Ferroelectric polymer compositions are suitable for providing either theparticle scattering colorant or the matrix material for a composite thatis electrically switchable from one color state to another. The termferroelectric polymer as used herein includes both homopolymers and allcategories of copolymers, such as random copolymers and various types ofblock copolymers. This term also includes various physical and chemicalmixtures of polymers. Poly(vinylidene fluoride) copolymers, such aspoly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), are preferredferroelectric polymer compositions. Additional copolymers of vinylidenefluoride that are useful for the composites of the present invention aredescribed by Tournut in Macromolecular Symposium 82, pp. 99-109 (1994).Other preferred ferroelectric polymer compositions are the copolymers ofvinylidene cyanide and vinyl acetate (especially the equal mole ratiocopolymer) and odd nylons, such as nylon 11, nylon 9, nylon 7, nylon 5,nylon 3 and copolymers thereof.

Other particle scattering colorants include those that are absorbingparticle scattering colorants. One preferred family of such absorbingparticle scattering colorants are colloidal particles of metals (such asgold, silver, platinum, palladium, lead, copper, tin, zinc, nickel,aluminum, iron, rhodium, osmium, iridium, and alloys, metal oxides suchas copper oxide, and metal salts). Preferably the particles are lessthan about 0.5 micron in average dimension. More preferably theparticles are less than about 0.1 microns in average dimension. In orderto achieve special coloration effects, particles are most preferred thatare less than about 0.02 microns in average dimension. Particles thathave colloid-like dimensions are herein referred to as colloidalparticles, whether or not colloid solutions can be formed. Particlesizes that are below about 0.02 microns are especially useful forobtaining a wide range of coloration effects from one composition ofabsorbing particle scattering colorant, since these small sizes canprovide particle refractive indices and absorption coefficient maximathat depend upon particle size. This size variation of the wavelengthdependent refractive index and absorption coefficient is most stronglyenhanced for particles that are sometimes referred to as quantum dots.Such quantum dot particles preferably have a narrow particle sizedistribution and an average particle size that is from about 0.002 toabout 0.010 microns.

Convenient methods for forming colloidal particles include the variousmethods well known in the art, such as reaction of a metal salt in asolution or the crystallization of materials in confined spaces, such assolid matrices or vesicles. Likewise, well-known methods for producingcolloidal particles can be employed wherein colloid size liquid or solidparticles dispersed in a gas or a vacuum are either reacted or otherwisetransformed into solid particles of desired composition, such as bycrystallization. As an example of formation of colloidal particles thatare useful for the present invention by solution reaction methods, notethat Q. Yitai et al. have described (in Materials Research Bulletin 30,pp. 601-605 (1995)) the production of 0.006 micron diameter zinc sulfideparticles having a very narrow particle distribution by the hydrothermaltreatment of mixed sodium sulfide and zinc acetate solutions. Also, D.Daichuan et al. have reported (in Materials Research Bulletin 30, pp.537-541 (1995)) the production of uniform dimension colloidal particlesof β-FeO(OH) by the hydrolysis of ferric salts in the presence of ureausing microwave heating. These particles had a rod-like shape and anarrow size distribution. Using a similar method (that is described inMaterials Research Bulletin 30, pp. 531-535 (1995)), these authors havemade colloidal particles of α-FeO having a uniform shape (anddimensions) that can be varied from a tetragonal shape to close tospherical (with an average particle diameter of about 0.075 microns).

Fiber-like particle scattering colorants having a colloid-like size inat least two dimensions are also preferred for certain inventionembodiments, especially where anisotropic coloration effects aredesired. One unusual method for forming very small fibers that can beused as particle scattering colorants is by the deposition of a materialwithin the confining space of a hollow nano-scale fiber. The particlescattering colorant can then either comprise the filled nano-scalediameter fiber, or the fiber of the filler that is obtained by removing(by either physical or chemical means) the sheath provided by theoriginal hollow fiber. The general approach of making such fibers by thefilling of nano-size hollow fibers is taught, for example, by V. V.Poborchii et al. in Superlattices and Microstructures, Vol. 16, No. 2,pp. 133-135 (1994). These workers showed that about 6 nm diameternano-fibers can be obtained by the injection and subsequentcrystallization of molten gallium arsenide within the 2 to 10 nmchannels that are present in fibers of chrysotile asbestos. An advantageof such small dimension particles, whether in fiber form or not, is thatthe quantum mechanical effects provide refractive indices and electronictransition energies that strongly depend upon particle size. Hence,various different coloration effects can be achieved for a particlescattering colorant by varying particle size. Also, high dichroism inthe visible can be obtained for colloidal fibers of metals andsemiconductors, and such high dichroism can result in novel visualappearances for articles that incorporate such fibers as particlescattering colorants.

Colloidal particle scattering colorants, as well as particle scatteringcolorants that have larger dimensions, that comprise an outer layer thatabsorbs in the visible are among preferred particle scattering colorantsfor use in high Δn embodiments. In such high Δn embodiments there is alarge refractive index difference between the particle scatteringcolorant and the matrix in the visible wavelength range. The reason forthis preference is that a very thin layer of a visible-light-absorbingcolorant on the outside of a colorless particle scattering colorant candramatically enhance scattering at the particle-matrix interface, whilenot substantially increasing light absorption. In order to achieve thebenefits of such particle scattering colorant configuration, it ispreferred that (1) the coating of the visible-light-absorbing coloranton the surface of the particle scattering colorant comprises on averageless than 50% of the total volume of the particles of the particlescattering colorant, (2) the average particle size of the particlescattering colorant is less that 2 microns, and (3) the refractive indexof the coating of the particle scattering colorant differs from that ofthe matrix in which the particle scattering particle is dispersed by atleast 10% at visible wavelengths. More preferably, the coating of thevisible-light-absorbing colorant on the surface of the particlescattering colorant comprises on average less than about 20% of thetotal volume of the particles of the particle scattering colorant andthe average particle size of the particle scattering colorant is lessthat 0.2 microns. Preferred applications of such surface-enhancedparticle scattering colorants are for polymer fibers, polymer films, andpolymer molded articles. A method for the fabrication of colloidalparticles containing a visible-light-absorbing colorant on the surfaceof a colorless substrate particle is described by L. M. Gan et al. inMaterials Chemistry and Physics 40, pp. 94-98 (1995). These authorssynthesized barium sulfate particles coated with a conductingpolyaniline using an inverse microemulsion technique. The sizes of thecomposite particles (from about 0.01 to 0.02 microns) are convenient forthe practice of the high Δn embodiments of the present invention.

Colloid particles can either be added to the matrix in the colloid-formor the colloid particles can be formed after addition to the matrix.Likewise, these processes of colloid formation and dispersion can beaccomplished for a precursor for the matrix, which is subsequentlyconverted to the matrix composition by chemical processes, such aspolymerization. For example, if the matrix is an organic polymer, suchas nylon, the metal colloids can be formed in a liquid, mixed with theground polymer, and heated above the melting point of the polymer toproduce nylon colored with particle scattering colorants. On the otherhand, either colloidal metal particles or a precursor thereof can beadded to the monomer of the polymer, the colloid particles can be formedin the monomer, and the monomer can then be polymerized. A precursor fora metal colloid can also be added to the polymer matrix and thecolloidal particles can be then formed in a subsequent step. Suchprocesses of colloidal particle formation and incorporation can befacilitated by using a melt, dissolved, gel, or solvent-swollen state ofthe polymer (or a precursor thereof) during colloid incorporation,colloid formation, or colloid formation and incorporation.Alternatively, high energy mechanical commingling involving a solidstate of the polymer (or a precursor thereof) can be used to accomplishcolloid incorporation, colloid formation, or colloid formation andincorporation.

The incorporation of colloidal size particle scattering colorants in thegel state of a polymer prior to the formation of said gel state into apolymer fiber provides a preferred embodiment of this invention. Forsuch process, the particle scattering colorant should preferably have arefractive index that is at least 10% different from that of the solidpolymer matrix of the fiber at a wavelength in the visible. The averageparticle size of the particle scattering colorant is preferably lessthan about 0.2 microns, more preferably less than about 0.08 microns,and most preferably less than about 0.02 microns. For particle sizes ofless than about 0.02 microns, the particle scattering colorantspreferably significantly absorbs in the visible. For the case where theparticle scattering colorant is substantially non-absorbing in thevisible, the polymer fiber preferably comprises an electronic transitioncolorant that is commingled with the particle scattering colorant in thegel state. Preferably this electronic transition colorant issubstantially a black carbon form, such as carbon black, and theparticle scattering colorant comprises an inorganic composition. So asnot to interfere with fiber strength, both the particle scatteringcolorant and optional electronic transition colorant particle used forthese fibers should have very small dimensions, preferably less thanabout 0.02 microns. Such invention embodiments solve a long standingproblem that arises for the coloration of high strength fibers that arespun in the gel state, such as high molecular weight polyethylene thatis spun from a mineral oil gel. This problem is that conventionalorganic dyes or pigments can interfere with the formation of highquality product from the gel state. An important example of a highstrength fiber product spun from the gel state is Spectra™ polyethylenefiber made by AlliedSignal. These fibers, which are gel processed athigh temperatures, are widely used for fishing lines, fishing nets,sails, ropes, and harnesses. The absence of satisfactory prior methodsfor achieving coloration has been a problem in the art.

Ultrafine metal particles suitable for use as particle scatteringcolorants can be located on the surface of much larger particles thatare themselves particle scattering colorants. Combined-particlescattering colorants of this form are also suitable for the presentinvention. Methods for the preparation of such particle scatteringcolorants, where metal particles are deposited on much larger polymerparticles, are provided by H. Tamai et. al. in the Journal of AppliedPhysics 56, pp. 441-449 (1995). As another alternative, colloidalparticle scattering colorants can be located within larger particlesthat, depending upon their dimensions and refractive index in thevisible (relative to the matrix) can additionally provide particlescattering coloration. In any case, the larger particles are referred toas particle scattering colorants as long as the included particles areparticle scattering colorants. In a preferred case, the colloidalparticles are metal or metal alloy particles in a glass matrix. Methodsfor obtaining colloidal copper dispersed in SiO₂ -comprising glass aredescribed in the Journal of Non-crystalline Solids 120, pp. 199-206(1990) and methods for obtaining silicate glasses containing colloidalparticles of various metals, including gold and silver, are described inU.S. Pat. Nos. 2,515,936; 2,515,943, and 2,651,145, which areincorporated herein by reference. These glasses containing colloidalparticle scattering colorants are transformed to particles, such as bygrinding of melt processes, and used as particle scattering colorants inembodiments of this invention. In such embodiments, these particlescattering colorants are preferably dispersed in a polymer matrix,thereby providing particle scattering coloration for articles consistingof the resulting polymer composite.

An advantage of this colloid-within-particle design of the particlescattering colorant is that the glass particles can stabilize thecolloidal particles with respect to degradation processes, such asoxidation. A second advantage is that high temperature methods can beused for forming the colloid in the glass, which could not be used forthe dispersion of the colloidal particles directly in an organic polymermatrix. A third advantage of the colloid-within-particle method is thatthe processes of colloid formation and dispersion are separated from theprocesses of dispersion of the particle scattering colorant in the finalpolymer matrix, which can provide improved process economics. As analternative to the melt synthesis of colloid-within-particle particlescattering colorants, such colorants can be synthesized by a method usedby K. J. Burham et al., which is described in Nanostructure Materials 5,pp. 155-169 (1995). These authors incorporated colloidal particles insilica by doping metal salts in the silanes used for the sol-gelsynthesis of the silicate. By such means they obtained Ag, Cu, Pt, Os,Co₃ C, Fe₃ P, Ni₂ P, or Ge colloidal particles dispersed in the silica.For the purposes of the present invention embodiment, colloidalparticles dispersed in silica can be ground into suitable particle sizesfor use as particle scattering colorants.

Instead of an inorganic glass, the particle containing the colloidparticles can be a polymer. It is known in the art to prepare films ofcolloidal dispersions of various metals in the presence of vinylpolymers with polar groups, such as poly(vinyl alcohol),polyvinylpyrrolidone, and poly(methyl vinyl ether). Particle scatteringcolorants suitable for the present invention embodiment can be obtainedby cutting or grinding (preferably at low temperatures) a polymer filmformed by solvent evaporation of the colloidal dispersion. Morepreferably, such particle scattering colorants can be formed byeliminating the solvent from an aerosol comprising colloidal particlesdispersed in a polymer-containing solvent. Particle scattering colorantsthat are either semiconductors or metallic conductors are amongpreferred compositions for use in polymer fibers. Such particlescattering colorants will generally provide significant absorption atvisible wavelengths. In such case it is preferred that the particlescattering colorant has an average diameter in the smallest dimension ofless than about 2 microns, the neat polymer matrix is substantiallynon-absorbing in the visible, and the minimum in transmitted visiblelight intensity for the particle scattering colorant is shifted by atleast by about 10 nm as a result of the finite particle size of theparticle scattering colorant. More preferably, this shift is at leastabout 20 nm for the chosen particle sizes of the particle scatteringcolorant and the chosen matrix material. For assessing the effect ofparticle size on the minimum of transmitted light intensity, a particlesize above about 20 microns provides a good approximation to theinfinite particle size limit.

For particle scattering colorant compositions that provide a singlemaximum in absorption coefficient within the visible range when particlesizes are large, another application of the standard transmitted lightintensity ratio enables the identification of preferred particlescattering colorants. This method is to identify those particlescattering colorants that have at least two minima in transmitted lightintensity ratio that occur within the visible wavelength range. Such twominima, possibly in addition to other minima, can result from either abimodal distribution of particle sizes, or differences in the minimumresulting from absorptive processes and scattering processes for amononodal distribution of particle sizes. If the particle scatteringcolorants are required for applications in which switchability incoloration states are required, it is preferable that these two minimaarise for a mononodal distribution in particle sizes. The reason forthis preference is that the switchability in the refractive indexdifference between matrix and particle scattering colorant can provideswitchable coloration if particle scattering effects are dominant.Mononodal and bimodal particle distributions, referred to above,designate weight-fraction particle distributions that have one or twopeaks, respectively.

For applications in which reversible color changes in response totemperature changes are desired, particular ceramics that undergoreversible electronic phase changes are preferred particle scatteringcolorants for the present invention. Such compositions that undergoreversible transitions to highly conducting states upon increasingtemperature are VO₂, V₂ O₃, NiS, NbO₂, FeSi₂, Fe₃ O₄, NbO₂, Ti₂ O₃, Ti₄O₇, Ti₅ O₉, and V_(1-x) M_(x) O₂, where M is a dopant that decreases thetransition temperature from that of VO₂ (such as W, Mo, Ta, or Nb) andwhere x is much smaller than unity. VO₂ is an especially preferredcolor-changing particle additive, since it undergoes dramatic changes inboth the real and imaginary components of refractive index at aparticularly convenient temperature (about 68° C.). The synthesis andelectronic properties of these inorganic phases are described by Specket al. in Thin Solid Films 165, 317-322 (1988) and by Jorgenson and Leein Solar Energy Materials 14, 205-214 (1986).

Because of stability and broad-band ability to absorb light, variousforms of aromatic carbon are preferred electronic transition colorantsfor use in enhancing the coloration effects of particle scatteringcolorants. Such preferred compositions include various carbon blacks,such as channel blacks, furnace blacks, bone black, and lamp black.Depending upon the coloration effects desired from the combined effectsof the particle scattering colorant and the electronic colorant, variousother inorganic and organic colorants that are conventionally used bythe pigment and dye industry are also useful. Some examples of suchinorganic pigments are iron oxides, chromium oxides, lead chromates,ferric ammonium ferrocyanide, chrome green, ultramarine blue, andcadmium pigments. Some examples of suitable organic pigments are azopigments, phthalocyanine blue and green pigments, quinacridone pigments,dioxazine pigments, isoindolinone pigments, and vat pigments.

The use of either electronic transition colorants that are dichroic or adichroic matrix composition can be used to provide novel appearances.Such novel appearances can result, for example, since the scattering ofparticle scattering colorants can display a degree of polarization.Preferential orientation of the dichroic axis is preferred, preferablyeither parallel or perpendicular to the fiber axis for a fiber or in thefilm plane for a film, and can be conveniently achieved byconventionally employed methods used to make polarizers, such asmechanical drawing. The dichroic behavior can be usefully developedeither in the same matrix component in which the particle scatteringcolorant is dispersed or in a different matrix component. One preferredmethod for providing dichroic polymer matrix materials for the large Δnembodiments is by incorporating a dye molecule in the polymer, followedby uniaxially stretching the matrix containing the dye molecule. Such adye molecule serves as a dichroic electronic absorption colorant. Theeffect of the mechanical stretching process is to preferentiallyorientate the optical transition axis of the dye molecule with respectto the stretch axis of the polymer. The creation of polarizing films bythe mechanical stretching of a polymer host matrix is described by Y.Direx et al. in Macromolecules 28, pp. 486-491 (1995). In the exampleprovided by these authors, the dye was sudan red and the host matrix waspolyethylene. However, various other combinations of dye molecules andpolymer matrices are suitable for achieving the polarizing effect thatcan be usefully employed in the particle scattering colorant compositesof the present invention embodiments.

Various chemical compositions that are capable of providingswitchability in refractive index or adsorption coefficients are usefulfor either host matrices, particle scattering colorants, or electronictransition colorants that enhance the effects of scattering particlecolorants. In order to achieve novel coloration effects that areanisotropic, all of these switchable chemical compositions that areanisotropic can optionally be incorporated in a preferentiallyorientated manner in fabricated articles. By providing refractive indexand electronic transition changes that occur as a function of thermalexposure, light exposure, or humidity changes, such materials (eitherwith or without preferential orientation) provide a switchablecoloration state. A host of such color-changing chemicals suitable forthe present invention are well known, such as the anils, fulgides,spiropyrans, and other photochromic organics described in the book by A.V. El'tsov entitled "Organic Photochromes" (Consultants Bureau, NewYork, 1990). Such color changing chemicals can be employed as electronictransition colorants that modify the visual effect of particlescattering colorants in polymer composites. Also, color changes inresponse to temperature, light exposure, or humidity can alternativelybe produced by using the many well-known materials that providerefractive index changes in response to these influences, and nosignificant change in absorption coefficients at visible lightwavelengths. Such materials can be used as either the matrix material orthe particle scattering colorants for the color changing composites.

A host of photopolymerizable monomers, photo-dopable polymers,photo-degradable polymers, and photo cross-linkable polymers are alsoavailable for providing the switchable refractive indices and switchableelectronic absorption characteristics that enable the construction ofarticles having switchable particle scattering coloration. Materialssuitable for this use are described, for example, in Chapter 1 (pages1-32) written by J. E. Lai in the book entitled "Polymers for ElectronicApplications", which is also edited by the same author (CRC Press, BocoRaton Fla., 1989). Improved materials that are now being introduced aredescribed by G. M. Wallraff et al. in CHEMTECH, pp. 22-30, April 1993.More exotic compositions suitable for the present application aredescribed by M. S. A. Abdou, G. A. Diaz-Guijada, M. I. Arroyo, and S.Holdcroft in Chem. Mater. 3, pp. 1003-1006 (1991).

Polymer colored articles of the present technology can also containfillers, processing aids, antistats, antioxidants, antiozonants,stabilizers, lubricants, mold release agents, antifoggers, plasticizers,and other additives standard in the art. Unless such additivesadditionally serve desired purposes as particle scattering colorants orelectronic transition colorants, such additives should preferably eitherdissolve uniformly in the polymer that contains the particle scatteringcolorant or such additives should have a degree of transparency and arefractive index similar to the matrix polymer. Dispersing agents suchas surfactants are especially useful in the present invention fordispersing the particle scattering colorant particles. Many suitabledispersing agents and other polymer additives are well known in the artand are described in volumes such as "Additives for Plastics", edition1, editors J. Thuen and N. Mehlberg (D.A.T.A., Inc., 1987). Couplingagents that improve the coupling between particle scattering particlesand host matrix are especially important additives for vanishing Δnembodiments, since they can eliminate fissure formation or poor wettingat particle-matrix interfaces. For cases where either a glass or aceramic is the particle scattering colorant, and the host matrix is anorganic polymer, preferred coupling agents are various silanes that arecommercially available and designed to improve bonding in compositesthat involve both inorganic and organic phases. Examples of suitablecoupling agents for particle scattering colorant composites of this typeare 7169-45B and X1-6124 from Dow Corning Company.

The colored articles of the present invention can optionally containmaterials that are either fluorescent or phosphorescent. An example ofsuch known materials are of the form Zn_(1-x) Cd_(x) S, where x is nogreater than unity, that contains Cu, Ag, or Mn impurities.

In various teachings of this invention we refer to photopolymerizablemonomers and oligomers. Examples of such compositions that are suitablefor the practice of invention embodiments are monomers containing two ofmore conjugated diacetylene groups (that are polymerizable in the solidstate), vinyl ether terminated esters, vinyl ether terminated urethanes,vinyl ether terminated ethers, vinyl ether terminated functionalizedsiloxanes, various diolefins, various epoxies, various acrylates, andhybrid systems involving mixtures of the above. Various photoinitiatorsare also useful for such systems, such as triarylsulfonium salts.

Various methods can be employed for the compounding and fabrication ofthe composites of the present invention. For example, particlescattering colorants can be compounded with polymeric matrix materialsvia (1) melt-phase dispersion, (2) solution-phase dispersion, (3)dispersion in a colloidal polymer suspension, or (4) dispersion ineither a prepolymer or monomer for the polymer. Films of the compositecan be either formed by solvent evaporation or by adding a non-solventto a solution containing dispersed ceramic powder and dissolved polymerfollowed by sample filtration, drying, and hot pressing. In method (4),the ceramic particles can be dispersed in a monomer or prepolymer thatis later thermally polymerized or polymerized using actinic radiation,such as ultraviolet, electron-beam, or γ-ray radiation. Particlescattering colorants can also be combined with the matrix byxerographic, power coating, plasma deposition, and like methods that arewell known in the art. For example, particle scattering colorants can beadded to fabrics or carpet by using xerography techniques described in"Printing Textile Fabrics with Xerography" (W. W. Carr, F. L. Cook, W.R. Lanigan, M. E. Sikorski, and W. C. Tinche, Textile Chemist andColorist, Vol. 23, no. 5, 1991). The coating of textile, carpet fiber,and wallpaper articles with particle scattering colorants in a fusiblepolymer matrix, so as to obtain coloration, is an especially importantembodiment because of the commercial importance of speedy delivery ofarticles that accommodate frequent style and color changes andindividual customer preferences. Such deposition can optionally bepreceded by a separate deposition of an electronic transition colorantin order to enhance the effect of the particle scattering colorant.

In order to obtain uniform mixing of the ceramic in the host polymer,ultrasonic mixers can be used in the case of low viscosity compositeprecursor states and static mixers and more conventional mixers can beused for melt blending processes. Static mixers, which are particularlyuseful for melt blending processes, are available commercially fromKenics Corporation of Danvers, Mass., and are described by Chen andMacDonald in Chemical Engineering, Mar. 19, 1973, pp. 105-110.Melt-phase compounding and melt-phase fabrication are preferred for thecompositions of the present invention. Examples of useful melt-phasefabrication methods are hot rolling, extrusion, flat pressing, andinjection molding. For the fabrication of the more complicated shapes,injection molding and extrusion are especially preferred.

In some cases it is desirable to achieve a degree of controlledaggregation of the particle scattering colorants in order to achieveanisotropy in coloration effects. Such aggregation to produce anisotropyin coloration is preferably in either one dimension or two dimensions,wherein the direction of such aggregation for different particleaggregates are correlated. Such correlation in aggregation is mostconveniently achieved by plastic mechanical deformation of a matrix thatis heavily loaded with the particle scattering colorant. For example,such mechanical deformation can be in the fiber direction for a fiber orin either one or both of two orthogonal directions in the film plane fora film. As an alternative to using particle aggregation to achieveanisotropy in coloration, anisotropy in particle shape can be used toachieve the similar effects. For example, mechanical deformation offilms and fibers during processing will generally cause plate-likeparticles to preferentially orient with the plate plane orthogonal tothe film plane and fiber-like particles to preferentially orient withthe particle fiber axis parallel to the fiber axis of the composite.

A special type of particle scattering colorant orientation effect isspecially useful for vanishing Δn embodiments. In such embodiments it isusually preferred that the particle scattering colorants and matrixmaterials are isotropic in optical properties. However, in order toobtain novel angle-dependent coloration effects, one can preferentiallyorient plate-like particles of an anisotropic particle scatteringcolorant in polymer films so that an optic axis of the particles isnormal to the film plane. Such particles and polymer matrix are chosenso that the ordinary refractive index (n_(o)) of the particles equalsthat of the matrix at a wavelength in the visible. Hence, a film articlewill appear highly colored when light perpendicular to the film plane istransmitted through the film. However, light that is similarly viewedthat is inclined to the film plane will be scattered at all wavelengthsso the article will appear either uncolored or less intensely colored.In such embodiments the particle scattering colorant is chosen to be onethat has the optic axis perpendicular to the particle plate plane, whichis the case for many materials having either hexagonal, trigonal, ortetragonal symmetry. Preferential orientation of the plane of theplate-like particles parallel to the film plane can be obtained byvarious conventional processes, such as film rolling processes, filmformation by solution deposition processes, and biaxial stretchingprocesses. Note that such plate-like particle scattering colorants arequite different from the plate interference colorants of the prior art.For these prior art colorants, no match of refractive indices of matrixand particle is require, and, in fact, large refractive indexdifferences between the particles and the matrix throughout the visiblecan increase the coloration effect.

Fibers of the present invention embodiments can either be formed byconventional spinning techniques or by melt fabrication of a filmfollowed by cutting the film into either continuous fibers or staple. Anelectronic transition colorant can be optionally included in thecomposite film composition. Alternately, a polymer film containing theparticle scattering colorant can be adhesively joined either to one sideor to both sides of a polymer film that contains an electronictransition colorant. The adhesive tie layer between these polymer filmlayers can be any of those typically used for film lamination. However,it is preferable to employ the same matrix polymer for the joined filmsand to select the tie layer to have about the same refractive index asthis matrix polymer. Alternately, the central film layer containingelectronic transition colorant and the outer film layers containing theparticle scattering colorant can be coextruded in a single step usingwell-known technologies of polymer film coextrusion. If the desired endproduct is a polymer fiber, these multilayer film assemblies can besubsequently cut into fiber form. Microslitter and winder equipment isavailable from Ito Seisakusho Co., Ltd (Japan) that is suitable forconverting such film materials to continuous fibers. Particularlyinteresting visual effects can be obtained if these fibers are cut froma bilayer film that consists of a polymer film layer containing theparticle scattering colorant on one side and a polymer film layercontaining an electronic transition colorant on the opposite side. Suchfibers that provide a different visual appearance for different viewingangles can be twisted in various applications, such as carpets andtextiles, to generate a spatially colored material due to the appearancein one viewing angle of alternating segments with different coloration.One coloration effect is provided if the fiber side that is in closestview is the particle scattering colorant film layer and anothercoloration effect is provided if the side that is in closest view is theelectronic transition colorant film layer. Such special colorationeffects of cut film fibers are most visually noticeable if the cut filmfiber strips have a width-thickness ratio of at least 5. Additionally,dimensional compatibility of such fiber for commingling withconventional polymer fibers in textile and carpet applications isincreased if the cut film fibers have a denier that is less than 200. Asan alternative to the slit-film process, either bilayer or multilayerfibers having these characteristics can be directly melt spun using aspinneret that is designed using available technology of spinnerets.This paragraph has emphasized the formation of fibers by the cutting ofpolymer films of this invention that provide particle scatteringcoloration effect. However, is should be emphasized that the films thathave been described also provide important commercial opportunities asfilm products for application in diverse areas, from product packagingon one extreme to wallpaper on another.

Sheath-core fibers which are suitable for the invention are fiberscomprising a sheath of a first composition and a core of a secondcomposition. Either the sheath or the core can be organic, inorganic, ormixed inorganic and organic, independent of the composition of the othercomponent. Preferably both the sheath and core of such fibers containorganic polymer compositions. Also, the particle scattering colorant ispreferably located in the sheath and an electronic transition colorantis preferably located in the core. By choice of either sheath or corecross-sectional geometry that does not have circular cylindricalsymmetry, it is possible to provide fibers that provide differentcolorations when viewed in different lateral directions. For example,the external sheath geometry can be a circular cylinder and the core canbe an ellipse having a high aspect ratio. When viewed orthogonal to thefiber direction along the long axial direction of the ellipse, theeffect of the electronic transition colorant can dominate coloration. Onthe other hand, a corresponding view along the short axis of the ellipsecan provide a visual effect that is less influenced by the electronictransition colorant. More generally, in order to achieve such angledependent visual effects the maximum ratio of orthogonal axialdimensions in cross-section for the outer surface of the sheath ispreferably less than one-half of the corresponding ratio for the core.Alternatively, the sheath and core should preferably both have a maximumratio of orthogonal axial dimensions in cross-section that exceeds twoand the long-axis directions in cross-section of sheath and core shouldpreferably be unaligned. Such fibers that provide a different visualappearance for different viewing angles can be twisted in variousapplications, such as carpets and textiles, to generate a spatiallycolored material whose appearance in one viewing angle is determined byalternating segments with different coloration.

The ability to change the coloration of sheath-core fibers by varyingthe relative cross-sections of sheath and core provides for theconvenient fabrication of yarns that display interesting visual effectsbecause of variations in the coloration of different fibers in the yarn.Such variation can be accomplished, for example, by varying the relativeor absolute sizes of the sheath and cores, their relative shapes, andthe relative orientation of the sheath and core cross-sections. For anyof these cases, the said variation can be provided either along thelength of individual fibers or for different fibers in a yarn.Preferably in these embodiments, the particle scattering colorant is inthe fiber sheath and an electronic transition colorant is in the fibercore. Also, a yarn consisting of such fibers is preferably assembleddirectly after spinning from a multi-hole spinneret. Variation in theindividual spinneret hole constructions, or variation in the feedpressures for the sheaths and cores for different fiber spinning holes,can permit the desired fiber-to-fiber variations in either sheathcross-section, the core cross-section, or both. Alternatively, variationin the coloration of individual fibers along their length can beachieved by convenient means. These means can, for example, be byvarying as a function of spinning time either (1) either the sheathpolymer feed pressure or the core polymer feed pressure or (2) therelative temperatures of the sheath and the core polymers at thespinneret. Of these methods, variation in coloration along the lengthsof individual fibers is preferred, and such variations are preferablyachieved by changing the relative feed pressures of the sheath and corefiber components. Such pressure variations are preferably accomplishedsimultaneously for the spinneret holes that are used to producedifferent fibers and such spinneret holes for different fibers arepreferably substantially identical. Yarns are preferably formed from thefibers at close to the point of spinning, so that correlation in thelocation of like colors for different fibers is not lost. As a result ofsuch preferred embodiment, the color variations of individual fibers arespatially correlated between fibers, so these color variations are mostapparent in the yarn.

The fact that fiber coloration depends upon both the sheath/core ratioand mechanical draw processes when the particle scattering colorant isin the sheath and the electronic transition colorant is in the coreprovides important sensor applications. These sensor applicationsutilize the coloration changes resulting from fiber wear and other fiberdamage processes, such as the crushing of fibers which can providecoloration by deforming the cross-sections of sheath and core, abrasionor fiber dissolution which can change the cross-section of the fibersheath, and fiber stretching (which can change the cross-sections ofsheath and core, provide particle scattering colorant aggregation, andincrease both polymer chain orientation and fiber crystallinity). In anycase, the basis for these color changes is generally a changing relativecontribution from particle scattering colorant and electronic transitioncolorant to article coloration. Such sensors can provide valuableindication of damage in articles such as ropes, slings, and tire cordwhere the possibility of catastrophic failure and uncertainties in whensuch failure might occur lead to frequent article replacement. Thesheath/core fibers of the present invention can be used either as acolor-indicating minority or majority fiber in such articles.

Special methods of this invention can be used to obtain particle-inducedcoloration for fibers that are spun in hollow form. The particles thatprovide coloration via scattering can be dispersed in a suitable liquid,which subsequently fills the hollow fibers. Optional electronictransition colorants can be included in this liquid in order to enhancethe coloration effect. This approach is enabled by using either aprecursor fiber that is staple (i.e., short open-ended cut lengths) orto use hollow fibers that contain occasional micro holes, where thehollow fiber core breaks to the surface. The existence of these microholes enables rapid filling of the fibers. Modest pressures ofpreferably less than 2000 psi can be used to facilitate rapid filling ofthe fibers. A low viscosity carrier fluid is preferably chosen as onethat can be either photopolymerized or thermally polymerized after thefilling process. As an alternative to this approach, the particlescattering colorant can be included in molten polymer from which thehollow fibers are melt spun. Then the polymerizable fluid that is drawninto the hollow fiber after spinning can include an electronictransition colorant for enhancing the coloration effect of the particlescattering colorant. Various modifications of these methods can beemployed. For example, melt spun fibers can contain various combinationsof particle scattering and electronic transition colorants, as can thefluid that is drawn into the hollow fibers. As another variation ofthese methods, hollow fibers spun from a melt that contain a particlescattering colorant can be coated on the interior walls with a materialthat absorbs part of the light that is not scattered by the particlescattering colorant. For example, such coating can be accomplished bydrawing an oxidant-containing monomer solution for a conducting polymer,solution polymerizing the conducting polymer onto the interior walls ofthe hollow fibers, and then withdrawing the solution used forpolymerization from the hollow fibers. The inner walls of hollow fibersare preferably colored with an electronic transition colorant using asolution dye process that requires thermal setting. For example, a dyesolution can be imbibed into the hollow fibers by applying suitablepressure, any dye solution on the exterior surface of the fibers can bewashed away, the dye coloration can be set by thermal treatment, and thedye solution contained within the fibers can be removed (such as byevaporation of an aqueous solution). As an alternative to thermalsetting, the setting of the dye on the inner surface of the hollowfibers can be by either photochemical or heating effects of radiation,such as electron beam, ultraviolet, or infrared radiation. Such thermalor photoassisted setting of the dye can be accomplished in a patternedmanner, thereby providing fibers that display the type of spatialcoloration effects that are sought after for carpet and textileapplications.

The same methods above described for obtaining internal wall dyeing ofhollow fibers can be used for the achievement of novel optical effectsvia deposition of particle scattering colorants on the inside of hollowfibers. These particle colorants are preferably deposited by imbibing acolloidal solution containing the particle scattering colorant into thehollow fibers and then evaporating the fluid that is the carrier for thecolloidal particles. The liquid in which the colloidal particles aredispersed can optionally contain a material that forms a solid matrixfor the colloidal particles after fluid components are eliminated. Suchcolloidal particle scattering colorants, whether deposited on the innerwalls as a neat layer or as a dispersion in a matrix, can then beoptionally coated with an electronic transition colorant by methodsdescribed above for coating the inner walls of hollow fibers that arenot coated with particle scattering colorants. Note that the abovedescribed deposition of colloidal particles on the inside of hollowfibers can result in aggregation of these particles to the extent thatthey transform from particle scattering colorants to electronictransition colorants. Depending upon the coloration effect desired, thisaggregation can be either desirable or undesirable.

In the following embodiment of this invention, particle scatteringcolorants are used in hollow fibers to produce photochromism. Suchphotochromism can be achieved using particle scattering colorants thatare photoferroelectrics. Preferred photoferroelectrics for thisapplication are, for example, BaTiO₃, SbNbO₄, KNbO₃, LiNbO₃, and suchcompositions with optional dopants such as iron. These and relatedcompositions are described in Chapter 6 (pp. 85-114) of"Photoferroelectrics" by V. M. Fridkin (Springer-Verlag, Berlin, 1979).Photovoltages of the order 10³ to 10⁵ volts can be generated forphotoferroelectrics, although it should be recognized that thesephotovoltages decrease as the particle size in the polarizationdirection decreases. The corresponding photo-generated electric fieldscan be used to reversibly produce aggregation (i.e., particle chaining)of photoferroelectric particles that are dispersed in a low conductivityliquid within the cavity of a hollow fiber. If these photoferroelectricparticles have suitably small dimensions, aggregation and deaggregationprocesses will provide a photo-induced change in the visual appearanceand coloration of the fiber. The electrical conductivity of the fluidcan determine the rate of return of the coloration to the initial stateafter light exposure ceases, since this conductivity can lead to thecompensation of the photo-induced charge separation that provides thephoto-induced field. Methods described above can be used for the fillingof the hollow fibers with the photoferroelectric-containing liquid, andsuch liquid can be sealed in the fibers by a variety of processes, suchas by periodic closure of the hollow tubes using mechanical deformation.Articles consisting of these photochromic fibers can be used for variousapplications, such as clothing that automatically changes color uponlight exposure.

In another invention embodiment, the particle scattering colorant is aphotoferroelectric that is dispersed in a solid matrix that has the samerefractive index as the photoferroelectric at some wavelength in thevisible (either when the photoferroelectric is not exposed to light orafter it has been exposed to light, or both). This embodiment uses thelarge refractive index changes that occur upon the exposure of aphotoferroelectric to light, which shifts the wavelength at whichrefractive index matching occurs (or either causes or eliminates suchrefractive index matching), thereby causing a coloration change inresponse to light.

In previously discussed embodiments of this invention (for sheath-corefibers, trilayer and bilayers films and derived cut-film fibers, andhollow polymer fibers), the use of particle scattering colorants in alayer that is exterior to the layer containing an electronic transitioncolorant has been described. One described benefit is the novelcoloration effects achieved. Another benefit of such configurations isparticularly noteworthy. Specifically, particle scattering colorantsthat provide blue coloration also generally provide significantscattering in the ultraviolet region of the spectra that can cause thefading of many electronic transition colorants. Hence, this ultravioletscattering can protect the underlying electronic transition colorantsfrom fading due to ultraviolet light exposure.

Preferred embodiments result from the advantages of using a particlescattering colorant to provide ultraviolet light protection forultraviolet-light sensitive fiber and film products. For articles inwhich the particle scattering colorant is dispersed in a first matrixmaterial that is substantially exterior to a second matrix componentcomprising an electronic transition colorant (such as for abovedescribed hollow fibers, sheath-core fibers, and trilayer films andderived cut-film fibers) it is preferred that (1) the first matrixcomponent and materials contained therein absorb less than about 90% ofthe total visible light that can be incident on the article from atleast one possible viewing angle, (2) the absorption coefficient of thefirst matrix component and materials contained therein is less thanabout 50% of that of the second matrix component and materials containedtherein at a wavelength in the visible, (3) and the particle scatteringcolorant is substantially non-absorbing in the visible. In addition, itis preferable that the first matrix component and materials containedtherein either absorb or scatter more than about 50% of uniformradiation at the ultraviolet wavelength at which the second matrixcomponent comprising the electronic dopant undergoes the maximum rate ofcolor fading. The term uniform radiation means radiation that has thesame intensity for all spherical angles about the sample. Uniformradiation conditions exist if there is the same radiation intensity forall possible viewing angles of the article. The average particle sizethat is most effective for decreasing the transmission of light througha matrix at a wavelength λ_(o) is generally greater than about λ_(o) /10and less than about λ_(o) /2. Hence, for maximum protection of anelectronic transition colorant that most rapidly fades at λ.sub.∞ theaverage particle for the particle scattering colorant should preferablybe from about λ_(o) /2 to about λ_(o)./10. Additionally, for thispurpose the particle scattering colorant should preferably beapproximately spherical (having an average ratio of maximum dimension tominimum dimension for individual particles of less than four) and thereshould be little dispersion in the sizes of different particles. Mostpreferably the average particle size for the particle scatteringcolorants used for ultraviolet light protection of electronic transitionpigments should be from about 0.03 to about 0.1 microns. Particlescattering colorants that are especially preferred for conferringultraviolet light protection for electronic transition colorants aretitanium dioxide and zinc oxide.

Materials suitable for the present art include inorganic or organicmaterials that have any combination of organic, inorganic, or mixedorganic and inorganic coatings. The only fundamental limitation on sucha coating material is that it provides a degree of transparency in thevisible spectral region if the entire surface of the article is coveredwith such a coating material. Preferred coating materials forapplication to film, fiber, or molded part surfaces are well-knownmaterials that are called antireflection coating materials, since theyminimize the reflectivity at exterior surfaces. Such antireflectioncoatings can enhance the visual effect of particle scatting colorants bydecreasing the amount of polychromatically reflected light.Antireflection coatings can be provided by applying a coating to thesurface of an article so that the refractive index of the coating isclose to the square root of the refractive index of the surface of thearticle and the thickness of the coating is close to λ/4, where λ is theapproximate wavelength of light that is most problematic. For example,antireflection coatings can be obtained by well known means for polymerssuch as polycarbonate, polystyrene, and poly(methyl methacrylate) byfluorination of the surface, plasma deposition of fluorocarbon polymerson the surface, coating of the surface with a fluoropolymer fromsolution, or in situ polymerization of a fluoromonomer that has beenimpregnated on the surface. Even when the refractive index of theantireflection polymer layer does not closely equal the square root ofthe refractive index of the surface of the article, light is incident atan oblique angle to the surface, and the wavelength of the lightsubstantially deviates from λ, antireflection properties suitable forthe present application can be obtained using such single layers.Furthermore, the known technologies of broadband, multilayerantireflection coatings can be used to provide antireflection coatingshaving improved performance. Hence, antireflection coatings can beprovided for essentially any substrate, such as a polymer film, thatdecrease the polychromatic surface reflection that can interfere withthe visual effect of particle scattering colorants.

The ability to arrange the light scattering particles in a patternedmanner is important for achieving the spatial coloration that isdesirable for many articles, such as polymer fibers. A number ofprocesses can be used to achieve such spatial coloration. One method isto use the effect of magnetic fields on ordering magnetic colloidalfluids, such fluids being transformable into solid materials by thermalor photochemical setting. Such thermal setting is preferably either bydecreasing temperature to below a glass transition or meltingtemperature or by thermal polymerization. Such photochemical setting ispreferably by photo-polymerization to a glassy state. Another usefulsetting process is solvent evaporation from the colloidal suspension.Such setting should be substantially accomplished while the magneticmaterial is in a magnetic-field-ordered state, so that novel opticalproperties are conferred on the article by scattering and absorptiveeffects of the ordered magnetic material. Examples of magnetic colloidalsuspensions that can be used to provide novel coloration effects areeither water-based or organic-based suspensions of nanoscale magneticoxides. Such suspensions, called ferrofluids, are obtainablecommercially from Ferrofluidics Corporation, Nashua N.H. and aredescribed by K. Raj and R. Moskowitz in the Journal of Magnetism andMagnetic Materials, Vol. 85, pp. 233-245 (1990). One example of howmagnetic particles can be deposited in a spatially variant way isindicated by returning to the above examples of hollow fibers. Suchhollow fibers can be filled with a dispersion of the magnetic particlesin a polymerizable fluid. The magnetic particles can be spatiallydistributed in a desired pattern along the length of the hollow fibersusing a magnetic field. Finally, the fluid can be polymerized orcross-linked thermally or by exposure to actinic radiation in order toset the structure. Polyurethane thermosets provide one preferred type ofthermally set fluid for this application.

Spatially variant coloration of fibers and films can be accomplishedquite simply by mechanical drawing processes that vary along the lengthof the fiber or film. Variation in the degree of draw can providevariation in the refractive index of the polymer matrix and the degreeof stretch-induced crystallinity. These variations provide spatiallydependent variation in the coloration resulting from particle scatteringcolorants. For such spatially dependent variation of coloration to bevisually perceived, predominant color changes should occur lessfrequently than every 200 microns, unless the separation between regionshaving different optical properties is sufficiently short to providediffraction grating or holographic-like effects.

Especially interesting and attractive visual effects can be achieved bythe deposition of particle scattering colorants as a pattern that isspatially variant on the scale of the wavelength of light. The result ofsuch pattering is the creation of a holographic-like effect. Thepreferred particle scattering colorants of the present inventionembodiment have refractive indices for all wavelengths in the visiblespectra which do not equal those of the host matrix at the samewavelength, which is in contrast with the case of Christiansen filters.In fact, it is preferable that the particle scattering colorants thatare patterned to provide the holographic effect differ from that of thematrix by at least about 10% throughout the visible region. Mostpreferably, this difference in refractive index of particle scatteringcolorant and host matrix is at least about 20% throughout the visibleregion of the spectra.

The effect of the particle scattering colorants on the coloration ofpolymer articles and the polymers contained therein can be dramaticallydecreased or even eliminated, which is an important advantage of thepresent technology--since it enables the recycling of originally coloredpolymers to provide polymer resin that has little or no coloration.Special embodiments of the present invention enable such recycling. Inthe first embodiment both particle scattering colorants and electronictransition colorants are employed in different matrix polymers, so thatthe coloration effect of the particle scattering colorant is substantialonly in the presence of the electronic transition colorant (whichabsorbs non-scattered light so that this light does not interfere withthe visual effect of light that is chromatically scattered by theparticle scattering colorant). In this embodiment, the scatteringcolorant has no significant absorption in the visible (or at least nosignificant absorption peak in the visible) and the matrix polymer forthe particle scattering colorant and electronic transition colorant aresufficiently different that separation by physical or chemical means isviable. For example, this separation can be accomplished by eitherdissolving only one of the matrix polymers or causing the matrix polymerfor the electronic transition colorant to depolymerize.

The second embodiment employs colored articles that preferably containonly a particle scattering colorant. In this type of recycling methodthe coloration of the polymer is either decreased or eliminated byeither (1) a thermal heating or irradiation process that eitherdecreases the refractive index difference between particle scatteringcolorant and the host matrix to a value that is small, but eithernon-zero anywhere in the visible range or substantially zero throughoutall the visible range; (2) a thermal heating or irradiation process thateither eliminates a match of refractive index between matrix andparticle scattering colorant at a wavelength in the visible or causessuch match to occur over a broad spectral range; or (3) either adissolution, evaporation, or chemical process that removes the particlescattering colorant from the host matrix. For example, the particlescattering colorant can be an organic composition that evidences a highrefractive index with respect to the matrix because of the presence ofdouble bonds. Chemical processes (such as ultraviolet-induced,four-centered coupling of double bonds to form cyclobutane rings) candramatically decrease the refractive index difference of the particlesand the matrix, thereby effectively eliminating the coloration. Asanother example, the particle scattering colorant can be chosen as onethat is sublimable at temperatures at which the matrix polymer isthermally stable, one that is soluble in solvents that are non-solventsfor the matrix polymer, or one that dissolves in the matrix polymer. Inall of these cases, the coloration of the polymer is either decreased oreliminated by either destroying the particles, decreasing the refractiveindex difference between the particles and the matrix, eliminating aperfect match of refractive indices of the particles and the matrix atonly one wavelength, or separating the particles from the matrixpolymer. In fact, methods above described for obtaining switchablecoloration of polymers (via refractive index changes), which are usefulfor obtaining spatial coloration effects in polymer articles, are alsouseful for either decreasing or eliminating coloration during recyclingprocesses. A third embodiment of this invention for providing recyclablecolored polymers uses mechanical processes, such as polymer grinding,that cause either aggregation or stress-induced chemical reaction of theparticle scattering colorant, thereby eliminating the effectiveness ofthe particle scattering colorants for providing coloration.

The particle scattering colorant embodiments of the present inventionthat are described above do not necessarily require the arrangement ofthe individual particles as an array having translational periodicity.Such arrangement is sometimes desirable, since novel visual appearancescan result, especially intense iridescent coloration. The problem isthat it has been so far impossible to achieve such periodic arrangementsin either the desired two or three dimensions on a time scale that isconsistent with polymer processing requirements, which are dictated byeconomics. The presently described invention embodiment provides aneconomically attractive method to achieve these novel visual effects forpolymers. The particle scattering colorants of this embodiment consistof primary particles that are arranged in a translationally periodicfashion in m dimensions, where m is either 2 or 3. At least onetranslational periodicity of the particle scattering colorants ispreferably similar to the wavelength of light in the visible spectrum.More specifically, this preferred translational periodicity is fromabout 50 to about 2000 nm. More preferably this translationalperiodicity is from about 100 to about 1000 nm. In order to obtain suchtranslational periodicity, it is desirable for the particle scatteringcolorant to consist of primary particles that have substantially uniformsizes in at least m dimensions. The particle scattering colorant canoptionally comprise other primary particles, with the constraint thatthese other primary particles are either small compared with the abovesaid primary particles or such other primary particles also haverelatively uniform sizes in at least the said m dimensions. The averagesize of the primary particles in their smallest dimension is preferablyless than about 500 nm.

The first step in the process is the preparation of translationallyordered aggregates of the primary particles. Since this first step doesnot necessarily occur on the manufacturing lines for polymer articles,such as fibers, films, or molded parts, the productivity of suchmanufacturing lines need not be reduced by the time required for theformation of particle scattering colorants consisting of translationallyperiodic primary particles. The second step in the process is tocommingle the particle scattering colorant with either the polymer hostmatrix or a precursor thereof. Then, as a third step or steps, anyneeded polymerization or crosslinking reactions can be accomplished andarticles can be fashioned from the matrix polymer containing theparticle scattering colorant particles. In order to optimize desiredvisual effects, it is critically important that such second and thirdstep processes do not completely disrupt the translationally periodicarrangement of primary particles within the particle scatteringcolorants. This can be insured in a number of ways. First, the averagesize of the particle scattering colorant particles in the smallestdimension should preferably be less than about one-third of the smallestdimension of the polymer article. Otherwise mechanical stresses duringarticle manufacture can disrupt the periodicity of the primary particlesin the particle scattering colorant. The particle scattering colorantdimension referred to here is that for the particle scattering colorantin the shaped polymer matrix of the polymer article. However, it is alsopreferable that the particle sizes of the particle scattering colorantin the fashioned polymer matrix of the polymer article are thoseinitially formed during the aggregation of the arrays of primaryparticles. The point is again that mechanical steps, such as mechanicalgrinding, should be avoided to the extent possible if these stepspotentially disrupt the translation periodicity within the particlescattering colorant, such as by the production of cracks or grainboundaries within the particle scattering colorant.

Various methods can be used for the first step of forming the particlescattering colorant particles containing translationally periodicprimary particles. One useful method is described by A. P. Philipse inJournal of Materials Science Letters 8, pp. 1371-1373 (1989). Thisarticle describes the preparation of particles having an opal-likeappearance (having intense red and green scattering colors) by theaggregation of silicon spheres having a substantially uniform dimensionof about 135 nm. This article also teaches that the mechanicalrobustness of such particle scattering colorant having a threedimensionally periodic arrangement of silica spheres can be increased byhigh temperature (a few hours at 600° C.) treatment of the silica sphereassembly. Such treatment decreased the optical effectiveness of theparticle scattering colorant, since the particles became opaque.However, Philipse taught that the particle aggregates recover theiroriginal iridescent appearance when immersed in silicon oil for a fewdays. Such treatment (preferably accelerated using either appliedpressure, increased temperature, or a reduced viscosity fluid) can alsobe used to produce the particle scattering colorant used for the presentinvention. However, it is more preferable if the mechanical robustnessis achieved by either (1) forming the translationally periodic assemblyof spherical primary particles from a fluid that can be latterpolymerized, (2) either imbibing or evaporating a fluid to inside theas-formed translationally periodic particle assembly and thenpolymerizing this fluid, or (3) annealing the translationally periodicparticle assembly (as done by Philipse), either imbibing or evaporatinga fluid in inside this particle assembly, and then polymerizing thisfluid. Alternatively, materials can be dispersed inside the periodicarray of primary particles by gas phase physical or chemical deposition,such as polymerization from a gas phase. Such methods and relatedmethods that will be obvious to those skilled in the art can be employedto make the particle scattering colorants that are used in the presentinvention embodiment. For example, the primary particles can be eitherorganic, inorganic, or mixed organic and inorganic. Likewise, theoptional material that is dispersed within the array of primaryparticles in the particle scattering colorants can be organic,inorganic, or mixed organic and inorganic. In cases where the particlescattering colorants would be too opaque to optimize visual colorationeffects if only gas filled the void space between primary particles, itis useful to use either a liquid or solid material in such spaces. Suchliquid or solid material can minimize undesired scattering effects dueto fissures and grain boundaries that interrupt the periodic packing ofthe primary particles. In such case, it is preferable if such fluid orsolid has a refractive index in the visible range that is within 5% ofthe primary particles.

Another method for providing useful particle scattering colorantsutilizes polymer primary particles that form an ordered array in polymerhost, which serves as a binder. Films suitable for the preparation ofsuch particle scattering colorants were made by E. A. Kamenetzky et al.as part of work that is described in Science 263, pp. 207-210 (1994).These authors formed films of three-dimensionally ordered arrays ofcolloidal polystyrene spheres by the ultraviolet-induced setting of aacrylamid-methylene-bisacrylamide gel that contained an ordered array ofsuch spheres. The size of the polymer spheres was about 0.1 microns, andthe nearest neighbor separation of the spheres was comparable to thewavelength of visible light radiation. A method for producing filmsconsisting of three-dimensionally ordered polymer primary particles thatdo not utilize a binder polymer is described by G. H. Ma and T. Fukutomiin Macromolecules 25, 1870-1875 (1992). These authors obtained suchiridescent films by casting an aqueous solution of monodispersedpoly(4-vinylpyridine) microgel particles that are either 250 or 700 nmin diameter, and then evaporating the water at 60° C. These films weremechanically stabilized by a cross-linking reaction that used either adihalobutane or p-(chloromethyl)styrene. Particle scattering colorantssuitable for the present invention embodiments can be made by cuttingeither of the above described film types so as to provide particles ofdesired dimensions. One preferred cutting method is the process used byMeadowbrook Inventions in New Jersey to make glitter particles frommetallized films. Various mechanical grinding processes might be usedfor the same purpose, although it should be recognized that lowtemperatures might be usefully employed to provide brittleness thatenables such a grinding process. For use as particle scatteringcolorants, it is preferably that the cutting or grinding process produceparticles that are of convenient dimension for incorporation withoutsubstantial damage in the host matrix, which is preferably a polymer.

The particle scattering colorants of this invention embodiment arepreferably formed in required sizes during the aggregation of primaryparticles. Any methods used for post-formation reduction in particlesizes should be sufficiently mild as to not interfere with the desiredperiodicity of the primary particles. Likewise, processing conditionsduring commingling of the particle scattering colorant in either thepolymer matrix (or a precursor therefore) and other steps leading to theformation of the final article should not substantially destroy theoptical effect of the periodic assembly of primary particles. Forparticle scattering colorants that are not designed to be mechanicallyrobust, preferred processes for mixing of particle scattering colorantand the matrix polymer (or a precursor thereof) are in a low viscosityfluid state, such as in a monomer, a prepolymer, or a solution of thepolymer used for the matrix. For such polymers that are not designed tobe mechanically robust, film fabrication and article coating usingsolution deposition methods are preferred for obtaining the particlescattering colorant dispersed in the shaped matrix polymer. Likewise,for such non-robust particle scattering colorants, polymer matrixformation in shaped form by reaction of a liquid containing the particlescattering colorant is preferred, such as by thermal polymerization,photopolymerization, or polymerization using other actinic radiations.Reaction injection molding is especially preferred for obtaining moldedparts that incorporate particle scattering colorants that are notmechanically robust.

In another embodiment of this invention the particle scattering colorantconsists of primary particles that are translationally periodic in twodimensions, rather than in three dimensions. Fiber-like primaryparticles having an approximately uniform cross-section orthogonal tothe fiber-axis direction tend to aggregate in this way when dispersed insuitable liquids. Likewise, spherical primary particles tend toaggregate as arrays having two-dimensional periodicity when deposited onplanar surfaces. For example, such particles can be formed on thesurface of a liquid (or a rotating drum) in a polymer binder thatadhesively binds the spherical particles into two-dimensional arrays.These array sheets can then be either cut or ground into the particlesizes that are desired for the particle scattering colorant.

For all of the above invention embodiment of particle scatteringcolorants that consist of translationally periodic primary particles, itis preferable for the volume occupied by the particle scatteringcolorants to be less than about 75% or the total volume of the matrixpolymer and the particle scattering colorant. The reason for thispreference is that the use of low loading levels of the particlescattering colorant can lead to improved mechanical properties for thecomposite, relative to those obtained at high loading levels. Asdescribed above for particle scattering colorants that are notaggregates of periodically arranged primary particles, the visual effectof the particle scattering colorants consisting of ordered arrays ofprimary particles can be enhanced using electronic transition colorants.Such means of enhancement, as well as methods for achieving color changeeffects that are switchable, are analogous to those described herein forother types of particle scattering colorants.

From a viewpoint of achieving coloration effects for polymer articlesthat are easily eliminated during polymer recycling, particle scatteringcolorants that consist of translationally-ordered primary particlearrays can provide special advantages, especially if the primaryparticles do not substantially absorb in the visible region and thepolymer article does not include an electronic transition colorant. Thereason is that processing steps that disrupt such arrays can greatlyreduce coloration effects. From this viewpoint of polymer recycling, itis useful to provide particle scattering colorants that are convenientlydisrupted by either thermal, mechanical, or chemical steps.

Various applications for which the compositions of this invention haveutility will be obvious to those skilled in the art. However, for sucharticles having switchable coloration or switchable transparency that isbroadband in the visible, more detailed descriptions of applicationsembodiments are provided in the following. One such application is inprivacy panels, windows, displays, and signs in which theelectric-field- induced switchability of the refractive index of aparticle scattering colorant, an electronic transition colorant, or oneor more matrix components provides either device operation or anenhancement of device operation. In one example type, the electric-field-induced-change in the refractive index difference between particlescattering colorant and the surrounding matrix component can be used tochange either (1) the transparency of an overcoating layer on a sign (sothat an underlying message is switched between visible and invisiblestates) or (2) the transparency of either a privacy panel or a window.For displays and signs, an electric field applied to a matrix layercontaining the particle scattering colorant can cause the degree ofparticle scattering to change--therefore changing the effective viewingangle for an underlying message (such as produced by a back-lightedliquid-crystal display or other types of static or changeableinformation-providing materials). The electric field can provideswitchable properties to either the particle scattering colorant, thematrix material for that colorant, an electronic transition colorant, orany other kind on information display material, or any combination ofthese materials.

Most preferably, the direction of a refractive index change provided bya particle scattering colorant (caused by an ambient influence, such asan applied electric field, temperature, time-temperature exposure,humidity, or a chemical agent) is in an opposite direction to that ofthe host materials. In this preferred case, the sensitivity of particlescattering to applied electric field or other ambient influence isenhanced by the refractive index change of both the particle scatteringcolorant and the matrix material for this colorant. Most preferably,such difference in the direction of refractive index change for particlescattering colorant and matrix material is for all possible lightpolarization directions. For the above applications, electric fields canbe applied in either patterned or unpatterned ways and differentelectric field can be applied to the particle scattering colorant andother materials, such as the electronic transition colorant. In generalthe local field that is across a particle scattering colorant in amatrix depends upon the state of aggregation of that colorant in thematrix, so a patterned variation in such degree of aggregation can beused to provide a patterned difference in the response of the particlescattering colorant to an applied electric field. For example, if theelectric conductivity and the dielectric constant of the particlescattering colorant are both larger than that of the matrix, anincreased voltage drop across the particle scattering colorant can beprovided by increasing the degree of particle aggregation. If theswitchability in particle scattering is largely a result of the electricfield influence on the particle scattering colorant, such aggregationcan increase the switchability.

Display or lighting devices that involve electroluminescent compositionsprovide special application opportunities. For example, theelectric-field-switching of particle scattering can be used to eitherchange the degree of diffuse light scattering from electroluminescentlight source or to provide a patterned distribution of light emission.In a preferred case, particles of the electroluminescent compositionserve as a particle scattering colorant Another application of thisinvention in the lighting area is for light bulbs and lighting fixturesthat slowly become transmissive after the light switch is pulled, whichis an application of thermochromic materials that is described in U.S.Pat. No. 5,083,251, which is incorporated herein by reference. Suchlight sources are sought after to provide natural time-dependentlighting effects reminiscent of the rising of the sun. An example ofsuch technology that uses a vanishing Δn embodiment is obtained byemploying a particle scattering colorant that at room temperature has arefractive index that is unmatched at any point in the visible with thatof the host matrix. The particle scattering colorant is selected so thatthe heating of the light source causes the refractive index or theparticle scattering colorant and the matrix to become matched in thevisible. Hence, the heating process eliminates particle scattering atthe matching wavelength, so that the light source becomes moretransmissive. If this matching is desired to be broadband, then thereshould be little dispersion of the refractive index difference betweenthe particle scattering colorant over the visible wavelength range. Insuch case where little dispersion in Δn is wanted, the particlescattering colorant and the matrix can be chosen so that the differencein n_(F) -n_(C) for the particle scattering colorant and the matrixcomponent is smaller in absolute magnitude than 0.0001. For thisapplication mode, it is most preferable if the match between therefractive index of the matrix and that of the particle scatteringcolorant is achieved discontinuously upon increasing temperature above adesired temperature as a consequence of a discontinuous phase transitionof either the particle scattering colorant or the matrix material.Otherwise, the color of the transmitted light will vary somewhatcontinuously with the temperature of the particle scattering colorantand associated matrix material.

Indicators devices for chemical agents, pressure, temperature, moisturepickup, temperature limits (such as freeze or defrost indicators), andtime-temperature exposure provide other applications opportunities forthe particle scattering colorants of this invention. For such devices,either reversibly or irreversibly switched coloration can result as aconsequence of switchability in either the refractive index or theelectronic transitions of either particle scattering colorants, matrixcomponents, or electronic transition colorants. For the mentionedtime-temperature indicators, a color change can indicate that either adesired thermal exposure has occurred (such as for product processing)or that an undesired thermal exposure has occurred (leading to undesireddegradation of a perishable product). Using the vanishing Δn embodiment,the wavelength at which a match in refractive index occurs betweenmatrix and particle scattering colorant can be a function of integratedthermal exposure. For example, polymer films that are used for thepackaging of frozen vegetables can undergo a color change when thevegetables have been suitably heated for consumption. As another exampleof the use of the vanishing Δn embodiment for indicating successfulprocessing, a resin that is undergoing set (such as circuit board) cancontain a particle scattering colorant in the setting matrix. Thechanging refractive index difference between the particle scatteringcolorant and the matrix then provides a color response that indicateswhen satisfactory resin set has occurred. A similar useful example ofthe application of particle scattering coloration (in the vanishing Δnembodiment) is for the indication of moisture pickup for polymers, suchas nylon 6--so as to avoid the unsuccessful processing that would occurif the polymer has too high a moisture pickup.

Particle scattering colorants of this invention also enable theconvenient labeling of articles, such as polymer films, using thethermal or photochemical changes in refractive index or electronictransitions that occur as a result of patterned laser beam exposureduring high speed product packaging operations. For example, numberswritten by a laser on a polymer film used for packaging can becomevisible as a result of light-induced changes in the refractive indexdifference between the particle scattering colorant and the matrix. Suchswitchable particle scattering colorant/matrix combinations can also beused as a signature in order to thwart product counterfeitingactivities. The particle scattering colorants can even be used forapplications where the switchability in refractive index match atultraviolet light wavelengths provides materials operation. For example,an intelligent sunscreen for bathers can be provided by dispersing aparticle scattering colorant in a fluid matrix that is initially matchedat ultraviolet solar wavelengths with that of the particles. Alight-induced change in refractive index of either the matrix or theparticle scattering colorant (so that refractive index matching nolonger occurs) can provide an enhanced effectiveness of the sun screenas a function of increasing solar exposure.

The particle scattering colorant embodiments of the present inventionare especially useful for the polymer articles formed by desk-topmanufacturing methods. The prior art technologies for desk topmanufacturing (which is also called rapid prototyping) are described inModern Plastics, August 1990, pp. 40-43 and in CHEMTECH, October 1990,pp. 615-619. Examples of such methods are various stereolithographytechnologies that involve either the patterned electron beampolymerization or patterned photopolymerization of monomers. In suchcase the particle scattering colorant and optional electronic transitioncolorant can be dispersed in the photomonomer-containing fluid. Inaddition to providing coloration, such materials can provide additionalbenefits of reducing shrinkage during resin cure. Vinyl ether oligomersand monomers, that are used in conjunction with triarylsulfonium salts,are especially preferred for these applications. This is theultraviolet-cured Vectomer™ system that has been developed byAlliedSignal. The particle size of the particle scattering colorant, aswell as other possible solid additives, should be sufficiently smallthat these particles do not settle appreciably during the fabrication ofan article. For this reason, particle scattering colorants that havecolloidal dimensions are particularly preferred. Another method forrapid prototyping is the Laminated Object Lamination Method in whichroll-fed sheets of polymer are cut by a soft-ware guided lightbeam--thereby building up the article one sheet at a time. In thismethod the particle scattering colorant and optional electronic colorantcan be either located in the polymer sheets, the adhesive that is usedto bind the sheets, or both. In another method used for rapidprototyping, thin layers of a powder are deposited on the surface of thearticle being constructed, and these layers are fused in a patternedmanner using a light beam. Alternatively, a binder (or a precursorthereof) is sprayed in a patterned manner on the powder (such as byusing ink-jet spraying), thereby enabling article shaping in threedimensions. As another alternative, the powder layers can be replaced bya squeegeed gel layer that is photoset in a patterned manner. In thesemethods, the particle scattering colorants and optional electronictransition colorants of presently described invention embodiments can beincorporated in the initial powders, the binder, the gel polymer, orcombinations thereof. Another technology for rapid prototyping buildsthree-dimensional articles by the patterned extrusion of thin coils ofpolymer. In such case, the particle scattering colorants and optionalelectronic colorants of the present invention can be additives to themolten polymer. In any of the above described technologies for rapidprototyping, material coloration can be obtained by using either thelarge Δn embodiment or the vanishing an embodiment of the presentinvention.

The following specific examples are presented to more particularlyillustrate the invention, and should not be construed as beinglimitations on the scope of the invention.

EXAMPLE 1

This example describes the achievement of blue coloration and an angledependent hue of shade by the melt spinning of trilobal fibers from amixture of a non-absorbing particle scattering colorant (which is a 35nm average diameter titanium dioxide) and an electronic transitioncolorant in a nylon matrix polymer. The colored fiber produced in thisexample consists of a commingled mixture of both the particle scatteringcolorant (titanium dioxide) and the electronic transition colorant(carbon black) in one nylon matrix. Unless otherwise noted, the nylonused in this and following examples is MBM, a nylon 6 produced byAlliedSignal Inc. Titanium dioxide particles of MT-500B (which is anuncoated titanium dioxide from Daicolor-Pope having an average particlediameter of 35 nm) were dry-blended at a 10% by weight concentrationwith dry nylon 6. The mixture was extruded, pelletized, and redried. The10% sample was dry-blended with more nylon 6, extruded, pelletized, andredried to a final let-down concentration of 1%. A carbon blackmasterbatch produced by AlliedSignal (containing 20% carbon black innylon 6) was let-down to a 1% carbon black concentration by dry-blendingthe master batch with nylon 6, extruding, pelletizing, and redrying themixture. The 1% titanium dioxide in nylon mixture was chip-blended withthe 1% carbon black in nylon mixture at a weight ratio of99.5/0.5,respectively. The resulting mixture was spun into fibers ofapproximately 50 μm outer diameter, drawn at a 3.2/1 draw ratio, andtexturized. The resulting fiber was a light blue to gray-blue with anangle-dependent hue in shade.

EXAMPLE 2

This example describes the modification of the Example 1 process by theuse of Caplube™, which is a vegetable oil based material that acts as adispersing agent for the titanium dioxide particle scattering colorant.The carbon black concentration in the nylon composite that is commingledwith a titanium dioxide/nylon composite is an order of magnitude lowerthan for Example 1. However, the amount of the carbon black/nyloncomposite that is commingled with the titanium dioxide/nylon compositeis correspondingly increased, so that the titanium dioxide/carbon blackratio obtained by commingling is essentially unchanged from that ofExample 1. As a result, the fiber coloration obtained in this example isessentially the same as for Example 1. Titanium dioxide particles ofMT-500B having an average diameter of 35 nm were milled with Caplubeovernight to a 40 weight percent paste of titanium dioxide in Caplube.The resulting mixture was dry-blended with dry nylon 6 to yield a finalconcentration of titanium dioxide of 1% by weight. This mixture was thenextruded, pelletized, and redried. A 0.1% by weight carbon blackcomposite in nylon6 was made in a similar way as described in Example 1for a 1% by weight carbon black concentration. The 1% titanium dioxidein nylon mixture was chip-blended with the 0.1% carbon black in nylonmixture at a weight ratio of 95/5, respectively. Fiber was spun, drawnand texturized from the resulting mixture using the process ofExample 1. The resulting fiber was a light blue to gray-blue with anangle-dependent hue in shade.

EXAMPLE 3

This example describes the achievement of a light blue to gray-bluecoloration and an angle dependent hue in shade by the melt spinning oftrilobal fibers from a mixture of a non-absorbing particle scatteringcolorant in nylon 6 and carbon black in polypropylene. While theparticle scattering colorant (titanium dioxide) and the electronictransition colorant were in separate matrices, these matrices werecommingled in a substantially interpenetrating manner by a melt mixingprocess. A 1% by weight composite of the MT-500B titanium dioxide innylon was made as described in Example 1. Carbon black was dry-blended,extruded, and pelletized in polypropylene (from Himont Co.) to a finalconcentration of 0.1 weight percent. The 1% titanium dioxide in nylonmixture was chip-blended with the 0.1% carbon black in polypropylenemixture at a weight ratio of 98/2, respectively. The resulting mixturewas spun into fiber, drawn and texturized using the process ofExample 1. The resulting fiber was a light blue to gray-blue with anangle-dependent hue in shade.

EXAMPLE 4

This example demonstrates that the coloration effect can besubstantially changed if the titanium dioxide/nylon mixture is thesheath and the carbon black/nylon mixture is the core of a trilobalsheath-core fiber. This example contrasts with that of Examples 1 and 2(where the particle scattering colorant and the electronic transitioncolorant were mixed together in the nylon) and with Example 3 (where theparticle scattering colorant is mixed in nylon 6, the electronictransition colorant was mixed in polypropylene, and these two polymermixtures were then commingled together prior to the spinning process).Titanium dioxide particles of MT-500B were dry-blended at a 10% weightconcentration with nylon 6 and then let-down with additional nylon 6 toa titanium dioxide weight concentration of 5%. A 1% by weightconcentration of carbon black in nylon 6 was made as described inExample 1. The titanium dioxide nylon blend (5% titanium dioxide) andthe carbon black nylon blend (1% carbon black) were spun into a trilobalsheath-core fiber yarn containing 64 filaments per fiber bundle. Thevolumetric sheath/core ratio was 60/40 and the individual fibers had amaximum diameter of about 50 microns. The resulting fiber yarn was navyblue. The yarn was subsequently drawn at a 3.2/1 draw ratio to produce a1305 denier yarn. The drawn yarn was then texturized. The drawn andtexturized yarn was a navy blue of a very slightly darker shade than theundrawn fiber.

EXAMPLE 5

This example shows the effect of changing from a trilobal sheath/corefiber with a volumetric sheath/core ratio of 60/40 in Example 4 to around sheath/core fiber with a volumetric sheath/core ratio of 70/30. Inboth this example and Example 4 the polymer matrix containing theparticle scattering colorant is exterior to the polymer matrixcontaining the electronic absorption colorant. A nylon composite with 5%by weight MT-500B titanium dioxide and a nylon composite with 1% byweight carbon black were made as described in Example 4. The 5% titaniumdioxide/nylon blend and the 1% carbon black/nylon blend were spun intoround sheath-core fibers that were combined to form a yarn having 128filaments per fiber bundle. The carbon black phase was in the core andthe titanium dioxide in the sheath in the individual fibers, which hadan outer diameter of about 50 microns. The volumetric sheath/core ratiowas 70/30. The yarn was navy blue, and was a slightly lighter shade thanfor the yarn in Example 4. The yarn was subsequently drawn at a 3.2/1draw ratio and texturized. The resulting drawn and texturized yarn was anavy blue of a very slightly darker shade than the undrawn fiber, but ofa lighter shade than the drawn and texturized fiber yarn of Example 4.

EXAMPLE 6

This example shows the combined effect of further decrease in thevolumetric sheath/core ratio (compared with that of Examples 4 and 5), adecrease in the carbon black concentration in the core, and a decreasein the titanium dioxide concentration in the sheath. A nylon compositewith 1% by weight MT-500B titanium dioxide was made as described inExample 1. A carbon black masterbatch containing 20% carbon black innylon 6 was let-down in two steps to a 0.03% by weight carbon blackconcentration. The 1% titanium dioxide/nylon blend and the 0.03% carbonblack/nylon blend were spun into round sheath-core fibers, that werecombined to form a fiber yarn containing 144 filaments. The carbon blackcontaining nylon was in the core and the titanium dioxide containingnylon was in the sheath in individual fibers having an average outerdiameter of about 50 microns. The volumetric sheath/core ratio was 95/5.The resulting yarn was pale blue/gray. The yarn color was significantlylighter than example 4 and 5. This yarn was subsequently drawn at a3.2/1 draw ratio, resulting in a drawn yarn that is 2637 total denier.The drawn yarn was texturized and processed into sleeves and carpet. Theresulting yarn was a blue/gray and had an angle-dependent hue in shade.The shade was a very slightly grayer shade than for the undrawn yarn,but of a significantly lighter shade than the drawn and texturized yarnof Example 4.

EXAMPLE 7

This example further shows the effects of changing the titanium dioxideconcentration, the carbon black concentration, and the sheath/core ratiofor sheath/core fibers. In this example the polymer matrix containingthe particle scattering colorant is substantially exterior to thepolymer matrix containing the electronic absorption colorant. Titaniumdioxide particles of MT-500B having an average dimension of 35 nm weredry-blended at a 10% weight concentration with nylon 6. The mixture wasextruded, pelletized, and redried. The 10% sample was dry-blended withmore nylon 6, extruded, pelletized, and redried to a final let-downgravimetric concentration of 6.2%. A carbon black masterbatch containing20% by weight carbon black in nylon 6 was let-down to a 1.54% by weightcarbon black concentration by dry-blending the master batch with nylon6, extruding, pelletizing, and redrying the mixture. The 6.2% titaniumdioxide/nylon blend and the 1.54% carbon black/nylon blend were spuninto trilobal sheath-core fibers, and these fibers were combined to forma yarn containing 64 filaments. The carbon black phase was in the coreand the titanium dioxide in the sheath for the individual fibers, andthe fiber outer diameter was about 50 microns. The volumetricsheath/core ratio was 80/20. The resulting fiber was blue with a lightpurple cast. The fiber color was lighter shade than in Example 5. Theyarn was subsequently drawn at a 3.2/1 draw ratio, texturized, andprocessed into sleeves and carpet. The resulting articles were mediumblue.

EXAMPLE 8

This example further shows the effect of changing the carbon blackconcentration and the sheath/core ratio for sheath/core fibers, ascompared with that for Examples 4 and 5. In this example the polymermatrix containing the particle scattering colorant is substantiallyexterior to the polymer matrix containing the electronic absorptioncolorant. Titanium dioxide particles of MT-500B having an averagedimension of 35 nm were dry-blended at a 10% weight concentration withnylon 6. The mixture was extruded, pelletized, and redried. The 10%sample was dry-blended with more nylon 6, extruded, pelletized, andredried to a final let-down concentration of 5% by weight titaniumdioxide. A carbon black masterbatch containing 20% carbon black in nylon6 was let-down to a 1.9% by weight carbon black concentration bydry-blending the master batch with nylon 6, extruding, pelletizing, andredrying the mixture. The 5.0% titanium dioxide/nylon blend and the 1.9%carbon black/nylon blend were spun into trilobal sheath-core fibershaving a maximum outer diameter of about 50 microns. These fibers werecombined to form a yarn containing 64 filaments that had a total denierof 3025. The carbon black phase was in the core and the titanium dioxidein the sheath. The volumetric sheath/core ratio was 65/35. The resultingyarn was a dark blue. The yarn color was lighter shade than in Example4. The yarn was subsequently drawn at a 3.2/1 draw ratio, texturized,and processed into a cut-pile carpet and woven sleeves. These articleswere a dark blue.

EXAMPLE 9

This example, which can be compared with Examples 4 and 5, shows how toobtain a very dark blue coloration. In this example the polymer matrixcontaining the particle scattering colorant is exterior to the polymermatrix containing the electronic absorption colorant. Titanium dioxideparticles of MT-500B having an average dimension of 35 nm weredry-blended at a 10% weight concentration with nylon 6. The mixture wasextruded, pelletized, and redried. The 10% sample was dry-blended withmore nylon 6, extruded, pelletized, and redried to a final let-downgravimetric titanium dioxide concentration of 3.8%. A carbon blackmasterbatch containing 20% by weight carbon black in nylon 6 waslet-down to a 0.46% by weight carbon black concentration by dry-blendingthe master batch with nylon 6, extruding, pelletizing, and redrying themixture. The 3.8% titanium dioxide/nylon blend and the 0.46% carbonblack/nylon blend were spun into trilobal sheath-core fibers having amaximum exterior diameter of 50 microns. The carbon black containingnylon was in the core and the titanium dioxide containing nylon was inthe sheath for the individual fibers. These fibers were combined toprovide a yarn containing 64 filaments that had a total denier of 2064.The volumetric sheath/core ratio was 50/50 for the individual fibers andthe color of these fibers was a dark navy blue. The yarn color wasdarker than in Example 4. The fiber was subsequently drawn at a 3.2/1draw ratio, texturized, and processed into a cut-pile carpet. Thiscarpet was a dark, navy gray/blue.

EXAMPLE 10

This example shows the effect of changing the particle size of thetitanium dioxide compared with that used for above examples that usetitanium dioxide as a particle scattering colorant, such as Example 8.In this example, although the titanium dioxide is claimed to have a"blue tone in whites and tints", the particle size is too large forinvention embodiments, and the titanium dioxide does not satisfy thecriteria for a particle scattering colorant in high Δn embodiments. ThisTiO₂ is here shown to produce a gray fiber. Titanium dioxide 555 fromWhittaker, Clark, and Daniels, Inc. was dry-blended at a 10% weightconcentration with nylon 6. The mixture was extruded, pelletized, andredried. The 10% sample was dry-blended with more nylon 6, extruded,pelletized, and redried to a final let-down concentration of 5%. Acarbon black masterbatch containing 20% by weight carbon black in nylon6 was let-down to a 1.9% by weight carbon black concentration bydry-blending the master batch with nylon 6, extruding, pelletizing, andredrying the mixture. The 5.0% titanium dioxide/nylon blend and the 1.9%carbon black/nylon blend were spun into trilobal sheath-core fibers thathad a maximum outer diameter of about 50 microns. The carbon black phasewas in the core and the titanium dioxide was in the sheath. Thevolumetric sheath/core ratio was 65/35. A yarn was formed by combining64 such fibers. This yarn was a medium gray and did not have a bluetone. The yarn was subsequently drawn at a 3.2/1 draw ratio, texturized,and processed into a cut-pile carpet. The resulting carpet was a mediumgray. Hence, the titanium was not effective in acting as a particlescattering colorant.

EXAMPLE 11

This example supports the demonstration of Example 10 that the use oftoo large a particle size for the titanium dioxide particles providesmaterials that do not have significant particle scattering coloration.As for Example 10, the titanium dioxide used was titanium dioxide 555from Whittaker, Clark, and Daniels, Inc. The difference between thisexample and Example 10 is that the trilobal carpet fibers had a sheaththat was 6.2 weight percent titanium dioxide in nylon, a core that was a1.54 weight percent carbon black in nylon, and the volumetricsheath/core ratio before drawing was 80/20. Other than the increase inthe particle size of the titanium dioxide, all preparative details arethe same as for Example 7. However, while the cut-pile carpet of Example7 was a medium blue, the carpet produced in this example was a lightgray.

EXAMPLE 12

This example demonstrates that, by producing a yarn in which differentfibers have a slightly different sheath/core ratio along their lengths,one can produce a yarn with multiple shades without having streaks ofcolor. In this example the polymer matrix containing the particlescattering colorant is exterior to the polymer matrix containing theelectronic absorption colorant. A nylon composite with 5% by weightMT-500B titanium dioxide and a nylon composite with 1% by weight carbonblack were made as described in Example 4. The 5% titanium dioxide/nylonblend and the 1% carbon black/nylon blend were spun into roundsheath-core fibers having diameters of about 50 microns. The carbonblack composite was in the core and the titanium dioxide composite wasin the sheath. The spinning pack was set-up to permit variation in thesheath/core ratio during the spinning of individual filaments. Thisvariation in sheath/core ratio was achieved by varying the pressuregenerated by the pump used to provide the sheath material stream for thefilaments, while keeping constant the pressure generated by the pumpused to provide the core material stream. The average volumetricsheath/core ratio was 90/10, although this ratio varied along the fiberlength according to the pressure provided by the pump for the corematerial. The different sections of the resulting yarn were differentshades of blue, ranging from an off-white up to a navy blue. The fiberwas subsequently drawn at a 3.2/1 draw ratio, and texturized. Theresulting yarn was multi-colored and contained individual filamentslengths having shades from an off-white to a navy blue.

EXAMPLE 13

This example shows how one can obtain various other colors forsheath-core fibers. In this example the polymer matrix containing theparticle scattering colorant is substantially exterior to the polymermatrix containing the electronic absorption colorant. The electronicabsorption colorant in this example is a red pigment called Fire EngineRed. The Fire Engine Red nylon 6 sample was obtained from AlliedSignaland contains a red pigment from Stanridge Color Company. A 6.2% byweight composite of MT-500B titanium dioxide in nylon 6 was made asdescribed in Example 7. The 6.2% titanium dioxide/nylon blend and theFire Engine Red/nylon blend were spun into trilobal sheath-core fibershaving a volumetric sheath/core ratio of 65/35 and a maximum diameter ofabout 50 microns. These fibers were combined at the spinneret to form ayarn containing 64 filaments. This yarn was purple. The red pigmentcontaining nylon composite was in the core and the titanium dioxidecontaining nylon composite was in the sheath of individual fibers. Theyarn was subsequently drawn at a 3.2/1 draw ratio, texturized, andprocessed into sleeves. The resulting sleeves were a shade of purple.

EXAMPLE 14

This example shows that zinc oxide can be used together with carbonblack to provide a blue coloration for nylon fibers. The nanosized zincoxide was obtained from Nyacol Company. Fibers were obtained by spinninga zinc oxide/nylon composite through an apparatus that had previouslybeen used for the spinning of carbon black/nylon composite. The resultwas the achievement of a blue coloration for the zinc oxide/nylon fibersas a result of the pickup of the carbon black/nylon composite that wasresidual in the spinning apparatus. Six pellets of 20% by weight carbonblack in nylon and six pellets of 10 ppm carbon black in nylon weremixed together and pushed through a melt indexer, which was used as thespinning apparatus. Then three successive 2-3 gram batches of 1% byweight of zinc oxide in nylon were pushed through the melt indexer togive a first pass, second pass, and third pass fiber samples. Thesefilaments were gray with a blue cast. The color of fibers from eachsuccessive pass was somewhat lighter than those for the previous passes.

EXAMPLE 15

This experiment demonstrates the coloration resulting from the extrusionof nylon from a mixture of a non-absorbing particle scattering colorantand an electronic transition colorant in a nylon matrix polymer. Theparticle scattering colorant in this example was titanium dioxide andthe electronic transition colorant was iron oxide. Titanium dioxideparticles of MT-500B having an average particle dimension of 35 nm weredispersed in nylon 6 to a 2% by weight blend. Iron oxide was dry-blendedwith nylon 6 to a 1% by weight blend. Ten grams of the 1% ironoxide/nylon sample, 495 g of the 2% titanium dioxide mixture, and 495 gof nylon 6 were dry-blended, extruded, pelletized, and redried to yieldpellets with overall concentrations of0.01% iron oxide and 0.99%titanium dioxide. The pellets were a bright, rosy pink with a lightbluish undertone.

EXAMPLE 16

This example serves as a comparison with example 15 and demonstrates thestandard effects expected for iron oxide. Iron oxide was dry-blendedwith nylon 6 to a 1% by weight blend. Ten grams of the 1% ironoxide/nylon sample was dry-blended with 990 g of nylon 6. The sample wasextruded, pelletized, and redried to yield pellets with overallconcentrations of 0.01% iron oxide. The pellets were a watery, dull,salmon pink.

EXAMPLE 17

This experiment serves as a comparison with Example 15 and 16 anddemonstrates the standard effects expected for iron oxide and a particlesize titanium dioxide that is not effective as a particle scatteringcolorant. For this example titanium dioxide 555 available fromWhittaker, Clark, and Daniels, Inc. was used. Note that, although thistitanium dioxide is used to produce the blue tone described in theliterature in white and tints, it is a larger particle size than theMT500-B and does not provide the novel optical effects seen in thisinvention (Example 15). Titanium dioxide particles of Whittaker 555 havea particle size such that a maximum of 0.01% is retained on a 325 meshscreen. The estimated average particle dimension is above 200 nm. TheWhittaker titanium dioxide was dispersed in nylon 6 to a 5% by weightblend Iron oxide was dry-blended with nylon 6 to a 1% by weight blend.Ten grams of the 1% iron oxide/nylon sample, 198 g of the 5% titaniumdioxide mixture, and 792 g of nylon 6 were dry-blended, extruded,pelletized, and redried to yield pellets with overall concentrations of0.01% iron oxide and 0.99% titanium dioxide. The pellets were a soft,pale pink with no bluish undertone.

EXAMPLE 18

This experiment, which is for comparison with Example 15, demonstratesthe achievement of a blue coloration for polymer pellets formed from amixture of a non-absorbing particle scattering colorant and anelectronic transition colorant in a nylon matrix polymer (where carbonblack serves as the electronic transition colorant and titanium dioxideserves as the particle scattering colorant). Titanium dioxide particlesof MT-500B having an average particle dimension of 35 nm were dispersedin nylon 6 to a 2% by weight blend. Carbon black was let-down to a 1% byweight blend from a 20% by weight masterbatch in nylon 6. Nylon 6, the1% carbon black, and the 2% titanium dioxide mixture were dry-blended,extruded, pelletized, and redried to yield nylon pellets having overallconcentrations of 0.005% carbon black and 0.25% titanium dioxide. Thepellets were a light blue with a gray cast.

EXAMPLE 19

This experiment serves as a comparison with example 18 and demonstratesthe standard effects expected for titanium dioxide and carbon black. Forthis example the titanium dioxide 555 that was obtained from Whittaker,Clark, and Daniels, Inc. was used. It is noted that although thistitanium dioxide is used to produce the blue tone described in theliterature in white and tints, it is a larger particle size than theMT500-B and does not provide the novel optical effects seen in thisinvention (Example 18). Titanium dioxide particles of 555 fromWhittaker, Clark, and Daniels, Inc. were dispersed in nylon 6 to a 2% byweight blend. Carbon black was let-down to a 1% by weight blend from a20% by weight masterbatch in nylon 6. Nylon 6, carbon black (1%), andthe 2% titanium dioxide mixture were dry-blended, extruded, pelletized,and redried to yield pellets with overall concentrations of 0.005%carbon black and 0.25% titanium dioxide. The pellets were a gray with noblue undertone.

EXAMPLE 20

This experiment demonstrates the achievement of a gray/bluemetallic-looking fiber by using a mixture of a non-absorbing particlescattering colorant (titanium dioxide) and an electronic transitioncolorant (carbon black) in a nylon matrix polymer. Titanium dioxideparticles of MT-500B having an average particle diameter of 35 nm weredispersed in nylon 6 to a 2% by weight blend. Carbon black was let-downto a 1% by weight blend from a 20% by weight masterbatch in nylon 6. Atotal of 417.5 g of 2% titanium dioxide, 83.5 g of 1% carbon black, and1503 g of nylon 6 were dry-blended, extruded, pelletized, and redried toyield pellets with overall concentrations of 0.04167% carbon black and0.4167% titanium dioxide. The pellets were a dark blue-gray. The pelletswere spun into a gray/blue metallic-looking fiber.

EXAMPLE 21

This experiment demonstrates the achievement of novel optical effectsand coloration that are modified compared with those of Example 20.These effects were again achieved using a mixture of titanium dioxideand carbon black in nylon. The titanium dioxide is a non-absorbingparticle scattering colorant and graphite is the electronic transitioncolorant. Titanium dioxide particles of MT-500B having an averageparticle diameter of 35 nm were dispersed in nylon 6 to a 2% by weightblend. Carbon black was let-down to a 1% by weight blend from a 20% byweight masterbatch in nylon 6. A total of 208.75 g of 2% titaniumdioxide, 41.75 g of 1% carbon black, and 1753.5 g of nylon 6 weredry-blended, extruded, pelletized, and redried to yield pellets withoverall concentrations of 0.0208% carbon black and 0.208% titaniumoxide. The pellets were a dark blue with gray undertones. The pelletswere spun into fibers producing a gray-blue metallic-looking fiber ofslightly brighter metallic color than Example 20.

EXAMPLE 22

This example is to illustrate the effects obtainable for an absorbingparticle scattering colorant. In this example gold particles were usedto confer a pink coloration to nylon. A gold colloid solution wasprepared by heating 95 ml of an aqueous gold chloride (III) solutioncontaining 5 mg of gold. When this solution reached the boiling point, 5ml of an aqueous 1% sodium citrate solution was added with rapidstirring. No color was visible at first. The scattering centers formedover a five-minute period. In this period the solution changed from agrayish-blue to a red. Five ml of this solution was added to 5 g offinely ground nylon and the mixture was heated in a 100° C. vacuum ovenovernight to drive off the water. The result was a pink nylon sample.This sample was extruded into fiber filaments that evidenced a pinkcoloration with some purplish-pink spots.

EXAMPLE 23

This example is for contrast with Example 22, and it shows that thecolloidal scattering colorant should be trapped in the nylon (ratherthan aggregated on the surface) in order to obtain the desiredcoloration effect for nylon. A red colored gold colloid solution wasprepared as described in Example 22. This solution was added to an equalamount of millimeter size nylon pellets and the mixture was heated to100° C. The water boiled off, leaving only gray pellets of nylon.

EXAMPLE 24

Similar in technique to Example 22, this example demonstrates thatcolloidal metals can be used to produce color in nylon 6. A gold colloidsolution was prepared by first heating 237.5 ml of an aqueous goldtrichloride solution (containing 0.005% gold) to boiling. Then 12.5 mlof a 1% aqueous trisodium citrate solution was added to the boiling goldtrichloride solution during rapid stirring. Over 30 minutes, as thesolution boiled, the color changed from very pale grayish blue to a verydeep red. Upon cooling, all 250 ml of this deep red colloidal goldsolution was added to 500 g of finely ground nylon 6 and thoroughlymixed to produce an even color dispersion. All of the water in thismixture was then evaporated by drying the mixture overnight in a vacuumoven at 110° C. The resulting intensely pink powder containedapproximately 0.0025% colloidal gold. This powder was then spun intopink fibers containing 12 filaments per fiber bundle. The fibers weredrawn at a 3:1 draw ratio to produce pale pink drawn fibers.

EXAMPLE 25

This example demonstrates the generation of particle scatteringcolorants during extrusion using metal salts. No trisodium citrate orcomparable non-polymeric reducing agent was added. 0.104 g of an aqueous5% solution (by weight) of AgNO₃ was diluted with 10 ml deionized water.The mixture was added to 30 g of ground nylon 6. The sample was mixed,placed into a vacuum oven, and held at 85° C. for 3.5 hours. Thetemperature was raised to 100-105° C. for two hours. The resulting whiteor off white powder was removed from the oven. This powder was then spunthrough a melt indexer to yield a bright yellow filament.

EXAMPLE 26

This example further illustrates the use of metal salts to produce colorin nylon. AuCl₃ (0.015 g) was dissolved in 3.75 ml of deionized water.Nylon 6 (7.5 g) was added and the mixture was blended, placed in avacuum oven and held at 85° C. for 3.5 hours. The temperature was raisedto 100-105° C. for two hours. The resulting light pink powder wasremoved from the oven. The powder was then spun through a melt indexerto yield a deep crimson filament.

EXAMPLE 27

This example demonstrates that hollow white polymer fibers containingtitanium dioxide particle scattering colorants become blue in appearancewhen these fibers are fluid filled with an electronic transitioncolorant Titanium dioxide particles having an average diameter of 35 nmwere dry-blended with dry MBM nylon 6 to obtain a 6.2% loading level ofthe titanium dioxide in the nylon. The mixture was extruded, pelletized,and redried. The samples were spun into hollow white fibers that werethen cut into staple. The fiber was put into a water solution containingnegrosin black. The black solution entered the fiber ends throughcapillary action, thereby producing a blue fiber.

EXAMPLE 28

This example evidences the extraordinarily high fade resistance of thecarpets and fibers produced using particle scattering colorants. Thecarpets and fibers from Examples 7 and 8, as well as commerciallyavailable pigmented nylon fibers (AlliedSignal) and commerciallyavailable dyed carpet (AlliedSignal) of similar colors, were put intoundiluted household bleach solutions (a 5.25% sodium hypochloriteaqueous solution) for 72 hours. The carpets and fibers from Examples 7and 8 evidenced no fading. On the other hand, the fibers and carpetsthat were either dyed or pigmented using conventional technology allfaded, such fading being most severe for the dyed fibers and carpets.

EXAMPLE 29

This example evidences the high fade resistance to ozone exposure ofwoven articles (sleeves) produced using particle scattering colorants.Sleeves from Examples 7 and 8, as well as sleeves made from commerciallyavailable pigmented nylon fibers (AlliedSignal) and commerciallyavailable dyed fibers (AlliedSignal) of similar colors, were put under astandard ozone test. The sleeves from Examples 7 and 8 had no visiblefading. Quite different results were obtained for the articles made byconventional technologies for producing coloration: the fibers from thesleeves of pigmented fibers had slight fading and the sleeves from dyedfibers had substantial fading.

EXAMPLE 30

This example shows that similar coloration effects as obtained inExample 8 result when the nylon fiber sheath in this example is replacedwith either a derivatized polypropylene sheath or a polyethyleneterephthalate sheath. Both in this example and in Example 8, theparticle scattering colorant in the fiber sheath was titanium dioxideparticles of MT-500B having an average diameter of 35 nm. Also, in allof these cases the fiber core was a carbon-black/nylon composite. Thischange in the sheath polymer did not appreciably change fibercoloration, which was dark blue.

EXAMPLE 31

This experiment demonstrates the method employed for determining whetheror not a candidate material has the properties required for use as aparticle scattering colorant for high Δn embodiments of this invention.A 0.001% by weight dispersion of the candidate particle scatteringcolorant was prepared in ethylene glycol. The transmittance of thedispersion was measured from 380 to 750 nm using a Perkin Elmer Lambda 7UV/Visible spectrophotometer, and the absorbance was calculated fromthis transmittance. The minimum absorbance (A_(min)) and maximumabsorbance (A_(max)) were identified in the visible wavelength rangebetween 380 and 750 nm. The required ratio was then calculated as theeffective absorbance at the absorbance maximum divided by the effectiveabsorbance at the absorbance minimum. The obtained results are listedbelow.

    ______________________________________                                                                        Particle                                          Scattering                                                                  A.sub.max /A.sub.min Material Colorant?                                     ______________________________________                                        3.06   MT-500B titanium dioxide (Daicolor-Pope)                                                               Yes                                             1.26 Standard particle titanium dioxide (1 um, Aldrich) No                    9.0 Colloidal silica (Nissan Chemical Industries, Ltd) Yes                    3.55 UV Titan P580 titanium dioxide (Kemira) Yes                              4.52 Tin oxide (5% solution, Nyacol, The PQ Corp) Yes                         18.14 Zirconia (10% solution, 100 nm size, Nyacol, Yes                         The PQ Corp)                                                                 1.55 Titanium Dioxide 555 (Whittaker, Clark and No                             Daniels)                                                                     3.31 MT-500 HD Coated Titanium dioxide Yes                                     (Diacolor-Pope)                                                            ______________________________________                                    

All of the above particulate materials are substantially non-absorbingin the visible. According to whether or not A_(max) /A_(min) is above orbelow 2, the investigated material is a particle scattering colorant forhigh Δn embodiments of this invention. With increasing values of thisratio above 2, the effectiveness of the particle scattering colorant forproducing coloration is generally increased.

EXAMPLE 32

This example evidences the high fade resistance to light exposure ofwoven articles (sleeves) produced using particle scattering colorants.Sleeves from Example 8 and 29 were exposed to a severe test oflightfastness: GM 112 KJ for 3 days. The sleeves had excellentlightfastness (delta ε) ratings of 0.87 for the nylon/nylon sleeve and0.79 for the PET/nylon sleeve. No fading was observed for the PET/nylonsleeve.

EXAMPLE 33

This example evidences the high bleach resistance of the color generatedusing particle scattering colorants. Fibers from example 23 were putinto an undiluted solution of household bleach (a 5.25% sodiumhypochlorite aqueous solution). There was no color fading evident after96 hours exposure to bleach solution.

What is claimed is:
 1. A fiber comprising a polymer matrix component inwhich particle scattering colorant particles are dispersed, wherein saidparticle scattering colorant particles comprise a metallic conductorselected from the group consisting of gold, platinum, copper, aluminum,lead, palladium, silver, rhodium, osmium, iridium, and alloys thereof;the particle scattering colorant particles have an average diameter inthe smallest dimension of less than about 0.02 microns; the polymermatrix component is substantially non-absorbing in the visible region ofthe spectrum; and the particle scattering colorant has a minimum in thetransmitted light intensity ratio in the 400 to 700 nm range that isshifted at least by 10 nm compared with that obtained for the samemetallic conductor having an average particle size above about 20microns.
 2. The fiber of claim 1 wherein the particle scatteringcolorant particles comprises one or more colloidal particles.
 3. Thefiber of claim 1, wherein the transmitted light intensity ratio has twominima in the wavelength region of the visible spectra and the particledistribution of the particle scattering colorant is mononodal.